Sean will join up at Franklin's Gardens in the summer.
Sean moved from Rotherham to Glasgow in 2003 and finished his first season by picking up the Glasgow Player of the Year trophy at the Scottish Rugby Union awards dinner. He made his international debut a year later against Samoa and has gone on to earn 12 caps to date.
Born in Perth, Sean played in 30 league and cup games for Glasgow last season and scored 12 tries. He has scored five tries so far this season, including one in every away game in pool three of the Heineken Cup.
Northampton coach Budge Pountney said: "We are delighted Sean has decided to join Northampton Saints. He has shown already this year what a raw talent he is in the Heineken Cup when he tore teams apart, including us, with his incisive runs.
"He is in the mould of a big physical winger with pace with a high work rate but also proves to be very elusive on the rugby pitch we are very much looking forward to welcoming him and working with him here at the Saints."
Sean became a huge favourite with the Glasgow supporters with his hard running and tough tackling. Everyone at glasgowwarriors.com wishes him the very best of luck with his new club - but hopefully he'll sign off with a few more tries wearing a Glasgow shirt.