Can I use the same account on Polkadot, Kusama and parachains?

Yes. In fact you can use the same account on most Substrate chains (but not all, since there are exceptions - check the video at the end of the article for a detailed explanation).

Your account uses the same public and private keypair for all chains in the background. Only the address format changes from one chain to the other. To learn more about the accounts and the address formats you can check this  wiki article and on the same page you can find conversion tools that convert between address formats on different chains.

To enable your account to be visible across all Substrate chains follow these steps.

If your account is in the Polkadot extension

1. Open the extension
2. Click on the three dots next to your account
3. Select "Allow use on any chain" in the drop-down menu
4. Refresh the Accounts page on Polkadot-JS UI

 

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If your account is on Polkadot-JS UI

1. In the Accounts page click on the three dots next to your account.
2. Disable the "only this network" switch

 

You will notice that a yellow warning mark appears next to your account, notifying you that the account is available on all chains.

 

IMPORTANT FOR LEDGER ACCOUNTS: You can allow a Ledger account on all chains only if it has been added directly on the Polkadot-JS UI. Ledger accounts added in the Polkadot extension cannot be used on all chains. However, it is strongly recommended that you use a Ledger account only on one network, otherwise you may lose access to your funds. The only exception are Kusama accounts which can be used on Statemine and vice versa.

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Should I use the same account across different chains?

That depends on what you're using your account for. Here are some advantages and disadvantages to help you decide:

Advantages

1. You don't have to create different accounts for each network, which means you don't have several mnemonic phrases and JSON files to keep track of.
2. It may be necessary to do it if you participated in crowdloans, in order to be able to manage your crowdloan rewards, since these are usually sent to the account you participated with.

Disadvantages

1. It can get confusing Tip: The account icon remains the same across all chains!
2. It creates a single point of failure. If your mnemonic phrase for your account is compromised, your tokens across all chains can be stolen.
3. Ledger accounts should be used only on one network! The only exception are Kusama accounts which can be used on Statemine and vice versa.

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We also have this educational video that sums up what's been discussed in this article: