Targett pipped James Ward-Prowse, Steven Davis and Shane Long to the award, denying Long the chance to win it twice in a row.
The Saints Academy graduate came into the side after defeat to Norwich City and helped Saints win three league games in a row.
That run of form coincided with a change of formation for Ronald Koeman’s men, as Targett lined up at wing-back.
The 3-5-2 formation saw Saints record three clean sheets in a row, shutting out Watford, West Bromwich Albion and Manchester United during that time.
“January was a massive month for us as a team and for me as an individual. I’m really happy to get the award.
“I think being part of the four nominees gave me lots of confidence. I didn’t think I’d actually win it so it has been great.
“I’d just like to say thank you to everyone who has voted and hopefully I can carry on the good form.”
The 20 year-old believes playing at as a left wing-back has helped him develop as a player in recent weeks.
“I’m enjoying it,” he affirmed. “When the manager said he wanted to change formation, he put Ryan at left-sided centre-back and me at wing-back.
“It gave me a lot of confidence and with Ryan behind me it gives me the belief to push on because I know he’s got my back.”