He can play again from February 13, meaning he will be available for Wales' trip to Ireland later that month. Henson's initial suspension ran until early March.
His likely comeback game will be for the Ospreys against Celtic League opponents the Borders in Swansea on February 17.
Henson was suspended for 10 weeks and two days after being cited and found guilty of elbowing Leicester prop Alex Moreno during a bitter European Cup clash between the Ospreys and the Tigers just before Christmas.
And although the ban has been cut, his absence from Wales' opening two Six Nations games will be a severe blow to coach Mike Ruddock, who has already lost injured players Kevin Morgan, Chris Horsman, Brent Cockbain, Tom Shanklin and Ryan Jones from his plans for this season's tournament.
The appeal committee was chaired by Brian McLoughlin, of the Irish Rugby Football Union, assisted by former Ireland and Lions prop Phil Orr and the Scottish Rugby Union's Peter Brown.
They accepted Henson's change of plea to guilty that he had struck Moreno with his elbow.
While the 23-year-old will be relieved to see a reduction, he is likely to still feel a huge degree of frustration at missing two key international matches.
Henson's late long-range penalty gave Wales an 11-9 victory over England in Cardiff last season, and he would have been an integral part of Ruddock's planning for the Twickenham trip - a ground where Wales have not won since 1988.
But Ruddock must now move on without his midfield ace, although there would appear every possibility that Henson could feature at some stage in this season's Six Nations tournament.
Henson last played for Wales in the Grand Slam-clinching victory over Ireland almost 10 months ago, having missed the entire autumn Test programme while he recovered from groin surgery.
Henson was accompanied at the hearing at European Rugby Cup Limited's central Dublin offices by Ospreys manager Derwyn Jones and solicitor John Morris.
Henson refused to comment as he left the hearing, but the Ospreys issued a brief statement.
"It is still disappointing, but at least there has been a reduction," said an Ospreys spokesman.
"We felt justified in appealing the initial ban. There could have been more sympathy for Gavin, but we have accepted the panel's decision."