The 20-year-old started his career at Bolton Wanderers before moving to Atherton Collieries.
During his time at Bolton, he spent eight years in their academy setup, making one senior appearance in 2019.
Fitzmartin is no stranger to the Northern Premier League, having enjoyed time on loan at Stalybridge Celtic during the 2021/22 campaign.
He is capable of playing anywhere across the front three, adding versatility to the Reds' forward line as they look to mount a play-off push next season.
He goes straight into contention ahead of the club's pre-season friendly against Barrow on Saturday 8 July.
Photo credit: Bolton Wanderers