The 21-year-old came through Northampton’s youth system and was a popular figure among supporters but struggled for game-time under Jon Brady and was released at the end of last season.
He had been weighing up his options as a free agent but has now reached an agreement with Boston, who narrowly missed out on promotion to the National League last season. Pollock scored two goals in five appearances during a loan spell with the Pilgrims in 2021/22.
He netted within 10 minutes on an action-packed debut against Kettering Town in February before marking his 21st birthday with a last-gasp winner at Guiseley in March.
"We are delighted to welcome Scott back on a permanent basis," said Boston manager Paul Cox.
"He really hit the ground running during his loan spell in February and March and scored two very important goals against Kettering and Guiseley."
Pollock will officially join the Pilgrims on July 1.