The 20-year-old, who had spells on loan at Crystal Palace and Coventry last season, has joined the Iron for a month and will bolster their attacking options.
Ian Baraclough's men head to Deepdale on the back of a six-match winless streak in all competitions.
Baraclough said of Sears: "He's a lad who first came to our attention a few years ago when he burst on to the scene with West Ham.
"We were presented with the chance to bring him in and we're delighted to have him on board. He's come through the ranks and scored a goal on his debut for West Ham."
Sears told Iron Player: "It feels good to be here.
"I got a phone call on Sunday night saying that Scunthorpe wanted to take me.
"As soon as I heard they were interested I wanted to come down and get going really.
"It went through on Monday and I travelled up on the night before completing the deal on Tuesday morning."