His two goals against Southampton on the final day of the month sent Newcastle United to a Wembley cup final for the first time in 24 years and capped off a fine few weeks which have been him become an ever-more crucial part of Eddie Howe's side.
Speaking on his award, Longstaff said: "The end of January was obviously a big moment for me with the two goals against Southampton and what they meant for the club.
"Scoring in front of the home fans in a big game with an incredible atmosphere was really special and something that will stay with me for a very long time.
"Throughout the month we had some big moments and results with a lot of games so to win this award and be recognised by our supporters is something that I really appreciate."
Voting for the FUN88 Player of the Month takes place after every match in the club's post-match email.
Longstaff got the most votes for the Southampton win at St. James' Park as well as the home victory over Fulham, and was a consistent performer as he played in all seven games in January.
To ensure you receive the post-match email - which includes highlights, interviews, stats from the game and more - make sure you have 'news' ticked within preferences in your Newcastle United account. If you don't have an account, you can sign up here.