The 21-year-old centre-half was in excellent form for the Pilgrims throughout the month as they climbed to joint top in Skrill North — and reached the second round of the FA Trophy in the process.
Piergianni headed crucial winning goals in the league against Colwyn Bay and the FA Trophy at Southport — in addition to a further goal against Redditch United.
The former Peterborough United trainee has scored five goals in 22 appearances since joining the Pilgrims in the summer from Corby Town.
He will be presented with his award at a forthcoming home fixture.