Assuming that you use brew on your macOS, converting a ppk file to pem file is relatively easy.

  1. Install putty on your mac
➜ brew install putty
  1. Generate the key
➜ puttygen existing_key.ppk -O private-openssh -o new_key.pem

Here are the options used in above command:

  • -O private-openssh : save the output as a SSH-2 private key
  • -o new_key.pem : save the output to new_key.pem

Full credit go to this blog post [1], where I had learned to convert a PPK to PEM file.