Rylov, who won gold in the 100 and 200 meter backstroke events at last year's Tokyo Olympic Games, was among eight top Russian athletes who appeared on stage at the rally in Moscow last week, according to Russian newspaper Novaya Gazeta. The rally commemorated the eighth year of Russia's annexation of Crimea -- which is deemed illegal by the Ukrainian government and not recognized in the West.
"Following his attendance at the Luzhniki Stadium in Moscow at the weekend, Speedo can confirm that it has terminated the sponsorship of Evgeny Rylov with immediate effect," the company said in a statement to CNN.
"We condemn the war in Ukraine in the strongest possible way and stand in solidarity with the people of Ukraine, our athletes and our teammates who have been impacted by the conflict."
"As part of this decision, any outstanding sponsorship fees will be donated to the United Nations High Commissioner for Refugees (UNHCR)," Speedo added.
Rylov, 25, competed in Tokyo as part of the Russian Olympic Committee due to Russia's ban from major international sporting competitions over doping non-compliance.
According to pictures on social media, Rylov wore his Olympic medals at the rally. He also sported, reports Reuters, the letter 'Z' on his outfit -- a letter used by Russian forces as an identifying symbol on their vehicles in Ukraine following Russia's invasion of the country. Some supporters of the war in Ukraine have also displayed the symbol.
In a statement to CNN, the International Swimming Federation (FINA) -- swimming's world governing body -- said: "FINA is deeply disappointed to note the reports regarding Evgeny Rylov's appearance at the Luzhniki Stadium during Friday's rally. We are investigating the matter further."
FINA is one of the few sports governing bodies still allowing Russian and Belarusian athletes to compete as neutrals, despite recommendations from the International Olympic Committee (IOC) for an outright ban.
FINA's policy, according to the website insidethegames, is to review each athlete on a "case-by-case" basis.
The governing body has withdrawn the FINA Order previously awarded to President Putin in 2014.
CNN also contacted the Russian Swimming Federation for comment but they did not immediately respond.