The London-based star has been a fixture in TV and film in recent decades, he played Detective Inspector Jimmy Perez in the popular Shetland detective series.
He described the independence referendum as a chance for voters in Scotland to have an “actual say” in how they would like to shape their country.
“For the first time since 1979 Scottish people have a chance to have an actual say in how they would like a democracy to be,” he said.
“For their families, friends, and anyone, and anything they care about. Snap their hands off. Vote Yes.”
Henshall won’t have a vote in the ballot next month because he lives in London. His endorsement coincided with a visit to Shetland, Orkney and the Western Isles by Deputy First Minister Nicola Sturgeon.