MetaMask is an Ethereum wallet that lets you connect to the Ethereum blockchain. MetaMask isn't limited to just Ether(ETH) and you can interact with any ERC-20 token on the Ethereum Blockchain. 

MetaMask is a popular choice for non-custodial Ethereum Wallets.

The wallet comes with a default list of tokens but unfortunately, USDT Tether is not included, despite it being the top ERC-20 token when it comes to Market Cap.

Tether USD is the top ERC-20 token listed on etherscan.io

Here is how to add USDT Tether to MetaMask:

1. Open MetaMask by clicking on the extension.

2. Click on ADD TOKEN and go to Custom Token:

3. Now we just need to fill in the Token Contract Address for USDT Tether and not worry about the Token Symbol and Decimals of Precision. MetaMask will automatically fill it. To find the Token Contract Address for USDT Tether, go to https://etherscan.io/tokens and click on Tether USD (USDT). 

4. On the Right Hand Side, you will see 'Profile Summary' and under it, the 'Contract' address. Click on it to copy. 

5. Paste the address in the Token Contract Address field in step 2 and just click next.

6. That's it. Now your wallet will list USDT tokens and you can send and receive USDT on your wallet address. You won't get the USDT icon but the default icon for unlisted tokens which is normal and a feature of MetaMask. 

That's it.