What is a sidechain? A sidechain is a separate blockchain with its own validators that connects to a main chain, such as Ethereum or Bitcoin, through a two-way bridge. This guide explains how sidechains differ from rollups and what an NFT project gains and risks when it launches on one.
What is a sidechain in blockchain terms?
A sidechain blockchain runs on its own, producing its own blocks and reaching agreement through its own consensus method, such as proof of authority or delegated proof of stake. The ethereum.org page on sidechains lists Polygon PoS, Gnosis Chain, SKALE and Metis Andromeda as examples on the Ethereum side.
A bridge with contracts on both chains links the sidechain to the main chain. When a user moves an asset, the bridge locks or burns it on the first chain and mints a matching token on the other chain, so the asset itself never travels between chains.
A sidechain does not inherit the main chain's security, and it does not post its transaction data back to Ethereum. If a majority of the sidechain's validators act together, they can approve fraudulent transactions. The validators also choose the block size and block time, and large, frequent blocks leave room for many transactions, so users do not compete for space and fees stay low.
Sidechain, rollup or validium
Marketing pages often use these terms loosely, but the difference matters to anyone who holds valuable NFTs.
| Type | Who secures it | Examples | What it means for NFTs |
|---|---|---|---|
| Sidechain | Its own validators and consensus | Polygon PoS, Gnosis Chain, SKALE; Rootstock and Liquid on Bitcoin | If the validators collude or the bridge is hacked, bridged assets are at risk. |
| Rollup (layer 2) | Posts transaction data and proofs to Ethereum | Arbitrum One, OP Mainnet, Base, ZKsync Era | Users can withdraw to Ethereum under fixed rules, even if the company that runs the rollup misbehaves. |
| Validium or optimium | Proofs or state summaries go to Ethereum, and a separate committee or network stores the transaction data. | Immutable X (StarkEx); Ronin since its May 2026 move to the OP Stack | Fees stay low, but users need that committee or network to publish the data before they can prove their balances and withdraw. |
Two networks in this table changed recently, so older articles may describe them wrongly. Ronin, the Axie Infinity chain, ran as a sidechain until May 2026, when it became an Ethereum layer 2 built on the OP Stack. L2BEAT lists Ronin as an optimium, which posts summaries of its state to Ethereum but stores its transaction data elsewhere, and Ronin stores that data on a network called EigenDA. Immutable announced in 2025 that it would merge Immutable X into Immutable zkEVM, so new projects should not build on Immutable X. The layer 2 NFT guide covers rollups in detail.
Why NFT projects launch on a sidechain
- Low cost per mint. Games and loyalty programs mint thousands or millions of low-value tokens, and small rollup fees add up at that volume. SKALE charges users no gas fees, and other sidechains let a project pay its users' fees.
- Control over the network. A dedicated sidechain can limit who deploys contracts, set its own block time and give the project's transactions priority.
- EVM compatibility. Most sidechains run the Ethereum Virtual Machine, so the same Solidity contracts, the ERC-721 and ERC-1155 standards, and tools such as Hardhat and Foundry work without changes.
- Smart contracts with Bitcoin. Rootstock runs EVM contracts, and Bitcoin miners mine its blocks at the same time, which is called merge mining. Users pay gas in RBTC, a token pegged one to one to BTC, and Rootstock gives Bitcoin-focused communities an option besides Bitcoin Ordinals.
The bridge is the main risk
An NFT minted on a sidechain lives on that sidechain, and to reach larger marketplaces or long-term storage on Ethereum, it must cross a bridge. Most bridges use lock and mint: the original token is locked in a contract on one chain, and a copy is minted on the other chain. Whoever controls the bridge contract controls every locked asset.
Two attacks in 2022 show the risk:
- In March 2022, attackers took control of five of the nine validator keys of the Ronin bridge and withdrew about $625 million in ETH and USDC.
- In June 2022, attackers compromised two of the five multisig keys of Harmony's Horizon bridge and took about $100 million.
An NFT project on a sidechain can reduce this risk in four ways:
- Name one chain as the home of each collection, and state it publicly. Two "official" contracts on two chains confuse buyers and marketplaces.
- Use the sidechain's official, audited bridge, not a custom bridge.
- If you build a bridge anyway, require a high signer threshold, set rate limits and add monitoring that can pause transfers.
- Keep provenance: the bridged token's metadata should name the original contract and token ID.
The cross-chain NFT marketplace guide explains messaging protocols and bridge designs.
Building an NFT marketplace on a sidechain
The marketplace software is the same as on any EVM chain: collection contracts, a marketplace contract for listings and offers, an indexer and a web app. The sidechain adds these tasks:
- Check wallet support. Add the network to browser wallets for users, or use embedded wallets so users never set up an RPC endpoint.
- Check indexer and RPC providers. Smaller sidechains may have no hosted indexer, so you run your own subgraph or event indexer.
- Check which marketplaces and aggregators list the chain, because those listings decide where secondary trading happens.
- Choose a gas model. Users can pay in the native token, or you can pay for them through a relayer or account abstraction.
- Build an admin panel for collection approval, fee settings and removal of fraudulent listings.
- Audit the contracts and any bridge integration before mainnet launch.
On Polygon PoS, the network gas token changed from MATIC to POL in September 2024, so fee and treasury documents should use POL. The Polygon NFT marketplace guide explains the change. Polygon also ran a separate network called zkEVM Mainnet Beta, which stopped producing blocks in July 2026. Polygon PoS kept running, so NFTs on Polygon PoS were not affected.
When a sidechain is the wrong choice
Buyers of high-value art and collectibles want a security model they trust, which today means Ethereum mainnet or a large rollup. Sidechains suit high-volume, low-value assets such as game items, tickets and loyalty badges.
Running your own sidechain needs validators, a block explorer, a bridge and RPC nodes, and only a large transaction volume pays for that work. Most projects spend less by deploying on an existing network.
Frequently asked questions
What is a sidechain in simple terms?
A sidechain is a separate blockchain linked to a main chain by a bridge. Users move assets to the sidechain for lower fees and faster confirmation, and the sidechain's own validators secure those assets, not the main chain.
Is Polygon PoS a sidechain or a layer 2?
Polygon PoS has its own validator set and posts periodic checkpoints to Ethereum. It does not post full transaction data to Ethereum, so ethereum.org classifies it as a sidechain.
Is a sidechain the same as a layer 2?
No. A layer 2 rollup posts its transaction data and proofs to Ethereum, so users can withdraw even if the company that runs the rollup fails. A sidechain relies on its own validators and bridge.
Can I move NFTs from a sidechain to Ethereum later?
Yes, through a bridge, if the collection is mapped on both chains. Choose the home chain at launch, because a later move complicates provenance and royalties.
Do sidechain NFTs work in MetaMask and other wallets?
On EVM sidechains, yes. Once the user or your app adds the network, ERC-721 and ERC-1155 tokens display in any wallet that supports that network.