The parents-to-be attended a screening of movie Buttercup Bill at the Curzon Soho cinema in London on Tuesday evening (1 September).
They posed alongside actress Sadie Frost and the movie's writer and director Emilie Richard-Froozan.
Josh and Tamsin confirmed their baby news in July, with a rep for Josh saying the couple are "thrilled" to be starting a family together. They've been together for around four years but keep their romance away from the spotlight.
They were last pictured in public together at Wimbledon, where they were seen greeting Prince William and Kate Middleton in the Royal Box.
US star Josh, 36, became a major Hollywood heartthrob thanks to movies like The Faculty, Halloween H20, Pearl Harbour and 40 Days and 40 Nights. He can currently be seen in the horror series Penny Dreadful.
Meanwhile, actress Tamsin, 26, rose to fame as schoolgirl Chelsea Parker in St Trinians and its sequel St Trinians II: The Legend of Fritton's Gold.
Josh and Tamsin met on the set of romance movie The Lovers, playing a married couple.