PSBT Explained: How Bitcoin Transactions Get Signed

You may have seen a wallet mention a PSBT — usually right when it hands a transaction to a hardware device or asks another signer to approve it. It stands for Partially Signed Bitcoin Transaction, and it's the quiet standard that lets one device build a transaction and a different, safer device sign it. You don't have to understand the format to use it, but knowing what it does explains a lot about how modern Bitcoin security actually works.

What a PSBT actually is

A PSBT is a standard file format for a Bitcoin transaction that isn't finished yet. It was defined in a Bitcoin Improvement Proposal called BIP-174, and its whole job is to carry a transaction — plus everything a signer needs to review and approve it — between different wallets and devices without losing anything in translation.

Think of it as a form that gets filled out in stages. One tool writes in what the transaction does: which coins it spends, where they go, and the fee. That form is still missing the signatures. A PSBT holds the half-done transaction so it can be handed off, signed elsewhere, and handed back — with each party adding only its piece.

The problem PSBTs solve

For a long time, building and signing a transaction happened in the same place, on the same device, with your private keys sitting right there. That's fine for a simple hot wallet, but it breaks down the moment you want your keys somewhere safer than the machine building the transaction. Two common situations need exactly that split:

• →A watch-only wallet builds; a signing device signs. A phone or desktop wallet that knows your addresses but holds no keys can assemble a transaction. It then needs a hardware wallet or an offline computer — where the keys actually live — to approve it.
• →Multiple people (or devices) have to sign. A multisig setup deliberately requires several keys before coins can move. Each key might be on a different device, in a different city, held by a different person.

In both cases a single device can't finish the job alone. Something has to package the unsigned transaction, move it to each signer, and collect the signatures back. A PSBT is that package — and because it's a shared standard, a transaction built in one wallet can be signed by a completely different brand of device.

How the flow works

The lifecycle of a PSBT is easiest to follow as three roles. A single wallet often plays all three behind the scenes, but they're distinct steps:

• →Create. A wallet builds the unsigned transaction — the inputs to spend, the outputs to pay, and the fee — and writes it out as a PSBT. No signatures yet.
• →Sign. The PSBT goes to each signer. A signer inspects it, confirms the amounts and destination on its own screen, and adds its signature into the PSBT. In multisig, this repeats until enough signatures are gathered.
• →Finalize & broadcast. Once it has all the signatures it needs, the PSBT is finalized into a complete, valid transaction and broadcast to the network — where it waits in the mempool and gets mined like any other.

Why this is safer

The security win is simple: your private keys never leave the signing device. The watch-only wallet that built the transaction never sees them. The PSBT that travels between devices doesn't contain them either — it carries the unsigned transaction and, after signing, the signatures, but never the keys that produced them.

Because the PSBT is just data, you can move it across an air gap — a physical separation between an online device and an offline one. Wallets pass PSBTs as a file on a USB stick or SD card, as a QR code scanned between a phone and a hardware wallet, or over a cable. The offline signer can approve a transaction without ever touching the internet, which is about as hostile an environment as you can put between an attacker and your keys.

Signing on a dedicated device also means you get to verify on that device. A good hardware wallet shows the destination address and amount on its own trusted screen before you approve — so even if the computer that built the PSBT were compromised, you'd see a wrong address before signing.

Where you meet PSBTs in everyday use

Most people use PSBTs without ever seeing the acronym. If you've sent Bitcoin from a hardware wallet, a PSBT almost certainly moved between your wallet software and the device. If you run multisig, PSBTs are how the signatures get collected. Wallets tend to hide the term behind friendly buttons like “confirm on your device” or “export for signing.”

You're most likely to see “PSBT” spelled out when you're deliberately working across tools — exporting an unsigned transaction from one app to sign it in another, or coordinating a multisig spend. That's the standard doing its actual job: letting independent pieces of software cooperate on one transaction. It doesn't change the transaction that lands on-chain — the same signed transaction could have come from a single all-in-one wallet — it just makes the safer, split workflows possible.

The bottom line

A PSBT is a Partially Signed Bitcoin Transaction: a shared format that lets a transaction be built in one place and signed in another, including offline and across multiple signers. It keeps your keys on the device that holds them, makes air-gapped and multisig setups possible, and doesn't affect what you pay in fees. If your wallet ever mentions one, that's usually a sign it's doing something right.

When it's time to actually send, set the right fee first with the SatSaver tracker — a PSBT handles the signing, but the rate you pay is still up to you. If a transaction gets stuck, see RBF vs CPFP.