Klaviyo SMS Marketing: Setup, Consent and Smart Sending

Klaviyo SMS marketing gets set up wrong less because the interface is hard and more because teams treat it as an email checkbox rather than a second channel with its own consent, its own number and its own sending rules. This guide covers the actual setup, in the order Klaviyo expects it: the sending number, consent capture, double opt-in, adding SMS to flows you’ve already built, and Smart Sending across both channels — plus the one step almost every team gets wrong on the way through.

What Do You Need Before Setting Up Klaviyo SMS Marketing?

Three things have to exist before the first SMS sends: a Klaviyo account with the SMS channel available for your region, a registered sending number, and a sign-up mechanism that captures SMS consent as its own event rather than borrowing it from an email form. None of these are optional prerequisites you can work around — a flow with an SMS action attached to a profile with no valid SMS consent simply won’t send, and Klaviyo’s own compliance gate is what stops it.

Knowing the goal before opening the settings screen matters too. SMS is not a replacement channel for email inside Klaviyo — it’s a second channel on the same customer profile, priced and governed separately, and this guide assumes you already have live email flows running that SMS will extend rather than duplicate.

How Do You Register a Sending Number in Klaviyo?

A sending number is registered under Settings > SMS, and the number type you pick decides how messages route and how quickly a new program can scale. Klaviyo offers different number types depending on account and region — a dedicated number that sends only your account’s traffic, and shared or short-code options where those are available — and the registration flow in Settings > SMS walks through whichever types apply to your account before you pick one.

Registering the number is the step that triggers Klaviyo’s own compliance checks on the account, which is why it belongs first: consent capture and double opt-in settings attach to a specific registered number, not to the SMS channel in the abstract. Skipping ahead to build sign-up forms before a number exists just means redoing that work once the number is live.

What decides which number type is right for a given account — expected message volume, region, existing sending history — is — metric to confirm at the general level; it depends on specifics current only inside your own Klaviyo account and Settings > SMS at the time you register, not on a rule that holds across every account.

How Do You Capture SMS Consent the Right Way?

SMS consent has to come from an action that specifically named SMS — an explicit SMS opt-in checkbox on a sign-up form, a keyword texted to your registered number, or a checkout-flow consent capture that names texting specifically — and Klaviyo records that as a distinct SMS Consent property on the profile, separate from email consent.

The most common sign-up mechanism is a Klaviyo form with a phone number field and its own SMS consent checkbox, built the same way you’d build any Klaviyo sign-up form, except the checkbox language has to describe texting specifically rather than reusing generic “sign up for updates” copy that a reviewer — or a subscriber disputing a message later — could read as ambiguous about which channel it covers.

A keyword opt-in works the same way from the other direction: a subscriber texts a word to your registered number, and that action itself is the SMS-specific consent event, distinct from anything they may have separately done on email. Either mechanism satisfies the same requirement — consent has to be traceable to an action that named SMS, not inferred from an unrelated email signup.

Can You Reuse Your Email List for SMS?

No. An email opt-in — even a double-confirmed one — is not SMS consent, because it never asked the subscriber about receiving texts. Building an SMS list means capturing that consent fresh, through a form, a checkbox or a keyword that names the channel, even for subscribers who have been on the email list for years. Treating an existing email list as an SMS list on day one is the single most common setup mistake, covered in full below.

How Do You Set Up Double Opt-In for SMS?

Double opt-in means a new subscriber has to reply to a confirmation text before landing in a sendable segment, and the setting lives on the sending number’s own compliance configuration inside Klaviyo rather than as a separate account-wide toggle. Whether double opt-in is required, optional, or automatically enforced depends on the number type, the region and current carrier and regulatory requirements, all of which change — confirm the current requirement directly in your account’s SMS compliance settings and with counsel before assuming a specific number is configured the way you expect.

What’s checkable without outside research is the mechanism: once double opt-in is active on a number, a subscriber who opts in through a form or keyword receives an automated confirmation text and has to reply before their SMS Consent property becomes usable in a live flow. Building or testing a flow before confirming this setting is on can produce a flow that appears to work in a preview but silently fails to send in production, because the profile never crossed the confirmation step.

How Do You Add SMS to a Flow You’ve Already Built?

Open an existing, live flow in Klaviyo’s flow builder and drag an SMS action onto the canvas at the point in the sequence where a text does something the email step near it doesn’t — a shorter, more time-sensitive nudge, a reminder that doesn’t need the layout an email carries. This is additive to a flow you already have, not a rebuild: the email actions already in the flow keep their own timing, and the SMS action gets its own delay, independent of the steps around it.

The action that matters most here is the condition gating it. Every SMS action inside a flow should check SMS Consent equals true before it’s allowed to fire, even though the flow itself may be entered by profiles who never opted into SMS at all — a flow entry trigger and an SMS consent check are two separate conditions, and conflating them is how a flow ends up trying to text someone who only ever opted into email.

A Shopify Plus team running several live flows — abandoned cart, browse abandonment, post-purchase, win-back — typically adds SMS to one or two of them first, rather than every flow at once, and watches opt-out behaviour on those before expanding. Abandoned cart is the flow most teams add SMS to first, because a short text often reaches a customer faster than an email does in the narrow window right after a cart is left; the abandoned cart flow itself, and what makes its timing work, is covered separately.

How Do You Turn On Smart Sending Across Email and SMS?

Smart Sending is configured under Settings > Smart Sending, and once it’s turned on for both channels, it holds back a message to any profile that already received one on the other enabled channel inside the suppression window you set. It does not touch consent, list membership, or flow logic — a profile stays eligible and stays in the flow; Smart Sending is a separate layer deciding not to send yet.

The overlap risk is real specifically because adding SMS into flows that already send email creates the exact scenario Smart Sending exists to manage: a profile that gets an abandoned-cart email at hour one and an abandoned-cart SMS at hour two, from the same trigger, on the same day, is the kind of double-send that drives opt-outs on both channels rather than just one. Turning Smart Sending on before publishing a flow with a new SMS action, not after, is what keeps that overlap from ever reaching a live subscriber.

Smart Sending’s suppression window is a setting you choose, not a fixed platform default — set it to match how close together your flows are likely to fire a message on each channel for the same trigger, and revisit it once SMS has been live long enough to see actual overlap in the send log rather than guessing at it upfront.

What’s the Step Most Teams Get Wrong?

The step almost every team gets wrong is treating SMS consent as inherited rather than captured. A team migrates or launches SMS, wants the list to look substantial on day one, and either bulk-imports phone numbers collected for order confirmations or sets SMS Consent to true for anyone already subscribed to email. Both produce a list that looks ready to send to and isn’t: SMS Consent has to come from an action that specifically named SMS, and a transactional phone number or an email opt-in was never that action.

Building the SMS action inside a flow before confirming double opt-in is actually active on the sending number is a second common mistake, closely related to the first. The flow looks correct in the builder, the condition logic is right, and nothing sends — or sends to an unconfirmed segment — because the number’s own compliance gate was never checked. Both mistakes share the same root cause: setting up SMS as if it were an email feature bolted onto an existing list, rather than a channel with its own consent chain that has to be built from the first sign-up form forward.

How Do You Verify Your Klaviyo SMS Setup Actually Works?

Send a real test message through a live flow to a phone number you control, and confirm three things in order. First, that opting in through your actual sign-up form or keyword sets the SMS Consent property correctly on that profile — check the profile directly in Klaviyo rather than assuming the form did its job. Second, that the flow’s SMS action fires only for a profile with that consent set, by testing with a second phone number that has SMS Consent set to false and confirming no message sends. Third, check the flow’s message log for a delivered status specifically, not just sent — sent means Klaviyo handed the message to the carrier; delivered confirms the carrier actually routed it to the handset, and the gap between the two is where a misconfigured number or a failed double opt-in shows up.

Once those three checks pass, watch the opt-out rate on the flow for the first few sends before expanding SMS to additional flows. A spike in opt-outs right after launch is usually the Smart Sending window being too narrow, an SMS action firing too close to its email counterpart, or a consent chain that wasn’t as clean as the profile count suggested.

Whether adding SMS to a given flow is worth the build and compliance overhead is a question worth answering with real numbers rather than instinct — the flow revenue calculator puts a figure on what your existing flows already earn, which is the baseline a new SMS action’s incremental lift gets weighed against before you build it.

Klaviyo SMS marketing is a lifecycle-flows problem before it’s an SMS problem. Klaviyo’s own benchmark data puts automated flows at 41% of email revenue across more than 183,000 brands (vendor-reported) — a figure that only covers the email side, and a reminder of how much revenue is already running through the flows an SMS action gets added into. The sending number, the consent chain, the double opt-in gate and Smart Sending all exist to protect that program, and the setup work here is only worth doing in service of flows worth protecting. That’s the same discipline a lifecycle flows engagement brings to any scaling brand adding a channel to a program that already works — get the sequencing and the guardrails right before volume makes a mistake expensive.