How to import a private key into Electrum

If you already have HD wallet

1. File --> New/Restore
2. Choose wallet name, click Next
3. Choose Standard wallet, click Next
4. Choose Import Bitcoin addresses or private keys, click Next
5. paste your private key, click Next
6. Choose your password, click Next

If you have normal wallet

1. Click Wallet --> Private Keys --> Import
2. Paste your private key and click Import

How to import a single private key into Bitcoin Core

Debug Window

1. Open Debug Window
2. Go to menu: /Help/Debug Window
   
3. Click on the tab - Console

If your wallet is encrypted

1. If your wallet is encrypted you must unlock it. If not just skip this section.
2. To unlock wallet, just type into the box at the bottom:
   walletpassphrase "YourPassphrase" 600
3. (The 600 means your wallet is unlocked for 10 minutes (600 seconds))
   

Run Import Command in Debug Window

1. In the console at the very bottom is a text entry box. In here enter:
   importprivkey yourPrivateKey "TheLabelThatIWant"
   
2. You now have to be patient. On a fast PC it takes 2 minutes to import, and during this time it looks like you application has hung. After waiting a few minutes you will see:
   
3. The End

(Optional) Check if Private Key is Imported

1. Once Imported you can check that you have the address by closing the Debug window and going back to your address book
2. You should see the address here:
   

How to import a single private key into MultiBit Classic

The easiest way:

1. Make sure you are using Multibit Classic client
2. Create a new wallet saved to a file, say, example.wallet
3. Export the private keys, WITHOUT A PASSWORD, to a file example.key.
4. Open up the key file in a text editor. There is a big comment at the top that describes the format. It is simply lines, each line containing the sipa format key - beginning with 5 - and the key creation date in a particular format. You will have an example in there to copy as a new wallet has one key in it.
5. Add in your new key and a date a bit before when you created it.
6. Import the example.key file back into the wallet.
7. MultiBit will then replay the blocks to get the transactions and work out the balance.

How to import a single private key into Blockchain.info wallet

1. log into your blockchain.info account
2. click SETTINGS
3. click Addresses
4. click + Import Bitcoin Address
5. click Import Address button
6. paste your private key
7. click Import