The Bolton-born midfielder, who turned 24 just before Christmas, signed a three-and-a-half-year deal earlier this morning, after passing his medical at the Amex yesterday evening.
He is the club's second new arrival of the window; joining striker Jon Obika who arrived on loan from Spurs earlier this month.
Head coach Oscar Garcia commented, "We are delighted to welcome Dale.
"He is a Brighton player, he will suit our style. He's the type we like and want to bring to this club: dynamic, energetic and technically good. His character is good and he will fit well with the excellent squad we already have here.
Oscar says Stephens suits Albion's style
"We know we are bringing a good player. Nathan has worked with Dale closely at Charlton and we have also done our analysis and research. I like what I have seen and I am sure he will suit our style, enjoy the way we want to play and help us.
"He has played many senior games already - and knows the Championship - but he is also still young, and is very hungry to succeed. I am sure he will prove an excellent addition to our squad."
Bolton-born Stephens came through the ranks at Bury before joining Oldham in 2008, spending three years with the Latics.
During his time at Boundary Park the midfielder played 73 games and found the net 12 times, while also spending two spells out on loan, one with Rochdale in 2009 and one with Southampton in 2011, before leaving the club permanently to join Charlton in June 2011.
Stephens has played in excess of 80 games for the Addicks during his two-and-a-half years with the club, and has already made 30 appearances this season, scoring four times.
Albion v Doncaster at the Amex on February 8th, 3pm kick off. Tickets available online, at the Amex Ticket Office or via the telephone on 08443271901.