Joshua, who holds the WBA, IBF and WBO belts, joined a march in his hometown of Watford, while triple Wimbledon champion Becker posted on social media to show his involvement in the big rally in London.
Becker, 52, donned a green mask and a black hoodie and cap as he joined the large singing crowd in Parliament Square.
The tennis champion, whose first wife of seven years, Barbara Feltus, had an African-American father, has long denounced racism.
“Of course, there is racism in sport because racism is part of our society,” he told CNN in 2012.
Joshua took a more active part in his local demonstration, addressing a united crowd who had gathered around him. “We are united against the virus, and the virus I am talking about is racism,” he said.
“As of today, we can no longer be proud, we can no longer sit and remain silent on senseless and illegal killings, the sneaky racism of another human being based solely on what? Their skin color . “
“We have to speak out at peaceful protests, just like today, so well done, Watford.”
Joshua, who is lined up for a world title unification bout against fellow Briton Tyson Fury, wore a knee pad and was on crutches during the protest. It was a “precautionary measure” after he felt a pinch in training, according to his spokesperson.