Bridlington-based Day, who has progressed up the levels of non-league football since being plucked out of local league football by former Boro boss Steve Kittrick in 2016, will add more steel to the Boro attack.
Boss Jono Greening is delighted with Day's arrival. He said: "Firstly, I want to say a big thank you to the Boro fans for backing the Boost The Boro campaign and raising funds. Their help has made this possible.
"We are absolutely delighted to have Jake join us. He is a player we have been tracking for a while now. He is somebody who wants to play for us and can help us achieve our goal of promotion.
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"Jake is an honest, hard-working striker, who is a team player and a goalscorer. He wants to continue to learn and to improve, myself and the rest of the staff are really looking forward to working with him."
Chairman Trevor Bull added: "I’m delighted to welcome Jake back to our club and I’m sure our supporters will also be excited and looking forward to seeing him at the Flamingo Land Stadium in our colours once again.
"Jono has been working for weeks to make this happen, these things are complex and never easy. So it’s safe to say we’re all pleased to get this deal done.
"This signing has been made possible by the funds raised by our fantastic fans donating to the "Boost the Boro" campaign and the Valley Bar Seadogs sponsored walk from Whitby to Scarborough.
"This is a great example of everyone working together to make our club better.
"Thanks everyone and let’s make sure we all give Jake a big Boro welcome on Saturday."