Haynes, 25, who agreed personal terms earlier this week, has the option of a further year and teams back up with manager Chris Kiwomya, the pair having played together at Ipswich.
Kiwomya said: "I think everyone was aware that one of our top priorities this summer was adding extra firepower to the squad and I knew very early on that Danny was the man that I wanted to bring in.
"It was far from straightforward, but then it never is when you're going after good players, they always have other clubs interested in them and that is reassuring in some ways.
"I know Danny well, having seen him play at the very start of his career and I think he's the perfect fit for Notts County, he's a player that has the ability to excite the fans and we're looking forward to seeing him in action."
Haynes, who represented England at under-19 level, has also played for Millwall, Bristol City and Barnsley.
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- Chris Kiwomya