Day, a World Championships winner with England Under-20s in 2013, only played eight games for Northampton.
The 22-year-old Academy graduate found himself pushed down the pecking order, sitting below Kahn Fotuali’i, Lee Dickson and Joel Hodgson.
And with former Pirates No.9 Tom Kessell arriving at Franklin’s Gardens this summer, Day would have found his game time even more limited.
However, Cornish Pirates director of rugby Ian Davies is delighted with his capture, expecting Day to flourish under player-coach Gavin Cattle’s guidance.
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“Alex is young man who has been part of a very strong set up at Northampton,” Davies said.
“He is keen to play and develop so with Gavin’s coaching guidance we look forward to seeing him in a Pirates shirt.”