A web based interface to the Bitcoin API JSON-RPC

Command: dumpprivkey

The dumpprivkey RPC returns the wallet-import-format (WIP) private key corresponding to an address. (But does not remove it from the wallet.)

Parameter #1—the address corresponding to the private key to get
Result—the private key

The P2PKH address corresponding to the private key you want returned. Must be an address corresponding to a private key in this wallet.
dumpprivkey "address"

Reveals the private key corresponding to 'address'.
Then the importprivkey can be used with this output

1. "address"   (string, required) The bitcoin address for the private key

"key"                (string) The private key

> bitcoin-cli dumpprivkey "myaddress"
> bitcoin-cli importprivkey "mykey"
> curl --user myusername --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "dumpprivkey", "params": ["myaddress"] }' -H 'content-type: text/plain;'