Ajaji was in his goal-scoring form in the month of March for Rotherham, finding the net five times from midfield in the month under review.
The 25-year-old netted a brace against Blackburn and Queens Park Rangers and a goal against Norwich in their only two wins from four games in March.
“It is a massive honour, and it is something I am really pleased to receive. I have to say though that the award is a team effort, I wouldn't be in the position to collect the award without my team mates," Ajayi said in a statement on Rotherham United's website.
“During the month we performed well in a number of games, picking up some vital points. It was a good all round team effort. I am also hugely grateful to the staff, who have been brilliant.
"It was a good month, and to be given the award ahead of some very stiff opposition is even more pleasing.
“The season is coming to a close now, but from a team perspective we are still fighting hard to keep this club in The Championship, and from a personal point of view this will also give me confidence going into the remaining games this season.”
The Nigeria international beat off competition from Norwich City's Emiliano Buendia, Leeds United's Pablo Hernandez and Sheffield United's Jack O’Connell to win his first Championship prize.
A trained defender, Ajayi has been deployed as a defensive midfield for the most part of the season.
Ajayi has played in all Rotherham's 41 league games this season and scored seven goals in total.