Deciphr Academy
Track 1 · Module 6 · Lesson 3 of 6 · 5 min read

What is the difference between a sidechain and a rollup?

Short answer

A sidechain is a separate blockchain with its own validators and rules, linked to Ethereum by a two-way bridge.1 Unlike a rollup, it does not post its data back to Ethereum, so it does not get Ethereum's security.1,2

Rollupruns transactionsposts its dataEthereumEthereum keeps the recordSidechainown validatorsbridgeEthereumIt keeps its own record
Fig. 3. A rollup posts its data to Ethereum. A sidechain keeps its own record and is only joined to Ethereum by a bridge.

What a sidechain is

A sidechain is a separate blockchain that runs independently of Ethereum. It connects to Ethereum through a two-way bridge.1 It picks its own consensus method, which is the way its validators agree on new blocks.1

Some sidechains are EVM-compatible. That means smart contracts written for Ethereum also work on them.1 ethereum.org lists Polygon PoS and Gnosis Chain among the projects that offer sidechains.1

Examples are not advice

We name networks only as examples of each type. Naming one is not a recommendation.

Fast and cheap, but at a price

Sidechains often use faster block times and bigger blocks, to get speed and low fees.1 That makes it harder to run a full node, which can leave a few large nodes securing the chain.1 With fewer nodes, the chance of validators working together to cheat, or of a takeover, goes up.1

The key difference: where the record lives

Rollups run transactions away from layer 1, then post the data to layer 1, where Ethereum's network agrees on it.2 Sidechains do not post their transaction data or state changes back to Ethereum.1

So each sidechain is responsible for its own security, and does not inherit Ethereum's.1 If enough of its validators turn dishonest, they can commit fraud.1

ethereum.org gives a simple test. Does the network store its data on Ethereum's main network? Projects that do are often called rollups.3 If the data is stored somewhere else, the project is independent, not a direct extension of Ethereum.3

Rollup Sidechain
Posts its data to Ethereum Yes2 No1
Where its security comes from Inherits Ethereum's4 Its own validators1
How it is linked to Ethereum Run by smart contracts on Ethereum5,6 A two-way bridge1
Main appeal Lower fees, while settling on Ethereum4 Low fees and fast transactions1

In between: validiums

A validium uses validity proofs, like a ZK rollup, but does not store its data on Ethereum.2 ethereum.org groups sidechains and validiums together: they talk to Ethereum through bridges, but do not get their security or their data from it.4

How tokens get onto a sidechain

Bridges between Ethereum and a sidechain do not physically move tokens across. They usually mint new tokens on one side and burn them on the other.1

Our viewFast and cheap is not the same as safe. For any network, ask two things: who could change the record, and what could you do about it?
Next

The next lesson looks at how bridges work.

Sources
  1. Sidechains. ethereum.org developer docs, updated 23 February 2026.
  2. Scaling. ethereum.org developer docs, updated 13 April 2026.
  3. Intro to Ethereum Layer 2: benefits and uses. ethereum.org, checked 22 October 2026.
  4. What is layer 2?. ethereum.org, updated 23 July 2026.
  5. Optimistic rollups. ethereum.org developer docs, updated 23 June 2026.
  6. Zero-knowledge rollups. ethereum.org developer docs, checked 22 October 2026.
← Rollups: optimistic and zero-knowledgeNext: How bridges work →

General education built from the publicly available sources listed above. Not financial, legal, tax or investment advice. Naming a company, project or regulator is not an endorsement either way. Last checked 2026-10-22. Terms and disclaimer.