The actor met up with Shriver, 59, and two of the couple's children, Katherine Schwarzenegger, 25, and Christina Schwarzenegger, 23, at the Montage hotel in Beverly Hills, California, for the big day.
The former couple were spotted leaving the hotel at the same time.
Shriver wore a navy eyelet dress for the outing, her hair half up in a matching clip as she carried a white bag with gold chain handles. Arnold kept it casual in a black short-sleeve polo, and wore round tortoiseshell sunglasses.
The couple, who also share sons Patrick Schwarzenegger, 21, and Christopher Schwarzenegger, 17, split in 2011. Shortly after announcing their separation, the actor publicly admitted to fathering a child with the couple's longtime housekeeper. Shriver then filed for divorce in July 2011 after 25 years of marriage.
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The Terminator: Genisys star also received birthday wishes from pal Joe Manganiello on Thursday.
"Happy Birthday to my friend @Schwarzenegger," Manganiello wrote on Twitter, sharing a funny photo of the pair. "Here's to many more great adventures!"
Manganiello and the Austrian actor first met on the set of their 2014 film Sabotage and have been close ever since (Schwarzenegger even delivered the toast at the Magic Mike XXL hunk's engagement party).