The choice of words jarred as memories remain fresh of the 2013 deadly bombing of the event in the Massachusetts city.
"Congrats, you survived the Boston Marathon!" Adidas said in the subject line of an email it sent to the 26,492 participants who finished the annual race Monday.
The email instantly set off a furor on social media, where many people published screen grabs of the email and denounced its offensive nature.
"We are incredibly sorry. Clearly, there was no thought given to the insensitive email subject line we sent Tuesday," it tweeted.
"We deeply apologize for our mistake."
On April 15, 2013, two brothers of Chechen descent, Tamerlan and Dzokhar Tsarnaev, planted pressure-cooker bombs near the Boston Marathon finish line, killing three people and injuring 264 others.
On Monday, Geoffrey Kirui and Edna Kiplagat led a Kenyan clean sweep at the 121st Boston Marathon, winning the respective men's and women's races.
The Boston Marathon is the world's oldest annual marathon and one of the most prestigious races over the distance of just over 26 miles (42 kilometers) on the athletics calendar.