Solomons, who arrived at Ravenhill at the beginning of the 2001-02 season on a three year deal, had been offered a two year extension to his current contract following success in the Celtic Cup in December.
However, following the announcement that Northampton coach Wayne Smith is set to link up with the New Zealand Rugby Football Union, Solomons has accepted the vacant position at the Saints.
Solomons said: "It is common knowledge that shortly after my arrival I prepared a document entitled "Vision Ulster" which set out my vision for the future of Ulster Rugby. I sincerely believe that the strategic goals which I set myself have been achieved."
"I have been most fortunate to secure a coaching position at Northampton, a proud club with a rich and illustrious history."
Northampton operations director John Steele said: "We carried out a thorough search for Wayne's successor and Alan has proved himself to be strong in all the areas."
"Although Alan's focus will remain with Ulster until the end of the season, he will be very much involved in moulding the Saints squad for next year."
But Solomons said he did not reach the decision with ease, "Whilst I look forward to a new and exciting challenge it is with somewhat of a heavy heart that I leave Ulster," he said.
"I have loved every single minute of my time here. The support I have had from the squad, the management team, the Ulster Branch, the IRFU and the public has been immense. Thank you for three wonderful years."
"The Northern Irish are the finest people I have ever met, and a piece of my heart will always be in "the wee country."
Ulster Rugby Chief Executive Michael Reid thanked Alan Solomons for his honesty and integrity he has shown over the past few months, however he added that with 13 weeks of rugby left this season, and 10 hugely important games for Ulster, that there was still work to be done, "A Celtic double really would be the icing on the cake for Alan."
Ulster were Heineken Cup Champions in 1999 when they beat Colomiers 21-9 at Lansdowne Road, the following year Nothampton Saints narrowly beat Munster 9-8 with three penalty goals from current England outside half Paul Grayson.