The 30-year-old actor greeted his sibling, 26-year-old Dylan Efron, with a hug at the finish line near John Hancock Tower.
Efron shared videos from the event on Instagram and Instagram Stories, including a clip of his reunion with Dylan after the race.
"'What's the point of all the finish lines we cross in life if there's no one there to celebrate with?' This used to get me down when I felt alone- There's many reasons BTW," the star captioned the post.
"Ima be at every [race] with you from here until forever, Brother. Congrats @deylanefron #marathon #bostonstrong #bostonmarathon #letsgo," he added.
“What’s the point of all the finish lines we cross in life if there’s no one there to celebrate with?” This used to get me down when I felt alone- There’s many reasons BTW❤️😎🙏 - Ima be at every 🏁 with you from here until forever, Brother. Congrats @dylanefron #marathon #bostonstrong #bostonmarathon #letsgo 🏃
Dylan said in a post on his own account that his girlfriend, Courtney King, and Efron's support at the marathon gave him the "motivation" to finish the race.
"First Boston marathon [check]," he wrote. "Cold, windy, rainy...but what a memorable day. @zacefron and @coourtking waiting for me at the finish line was all the motivation I needed."
Dylan completed the marathon with a finish net time of 2:57:40 and a finish gun time of 3:00:55. Desiree Linden, an American, was the women's champion, while Japan's Yuki Kawauchi led the men.