Pears, 19, has been a regular in Middlesbrough’s under-23 team this season. He joined the Middlesbrough Academy when he was 9, and was a regular with the under-18s before joining the under-23s.
He has also been capped for England under-19s, making his debut in October 2016 against Holland.
His father, Stephen, made over 400 appearances for Middlesbrough and won four promotions in his career.
Darlington manager Tommy Wright told the club`s official website: “Aynsley has done very well in his career so far, and is held in very high regard. He is a promising keeper, who played against us in the friendly before Christmas. He has been capped for England under-19s, and trained with the England keepers at St George’s. If he has half the success in his career that his dad enjoyed, then he’ll be a very good keeper.
“I must thank Middlesbrough and particularly Craig Liddle, who has been a great help in helping us sign him on loan.”
As one keeper joins, another leaves, with Ed Wilczynski signing for Evo-Stik League Premier side Shaw Lane AFC on a permanent basis after spending the last month on loan with the Ducks.
Pears will go straight into the squad for Saturday’s home game with Chorley.