Hillary Clinton jokingly held up a Russian hat while delivering a graduation speech at Yale.
While she never referred to Trump by name, it was an apparent suggestion that Russia was behind Trump's victory in the 2016 election.
It was part of a university tradition in which students and faculty wear over-the-top hats in an otherwise serious ceremony.
Hillary Clinton poked fun at Donald Trump by exhibiting a Russian hat at Yale's graduation ceremony.
The former presidential candidate donned the hat while speaking on Sunday as part of Yale College's graduation events, also known as "Class Day." One university tradition on Class Day involves students and faculty wearing silly hats.
Clinton, an alumna of the university, told students during her address: "I see looking out at you that you are following the tradition of over-the-top hats. So I brought a hat too: A Russian hat.
"Look, I mean — if you can't beat them, join them," she quipped, in a subtle suggestion that Russia was behind Trump's victory in the 2016 presidential election.
Clinton never wore that hat, but held it near her head for about 10 seconds.
Watch the exact moment at the 32 minute 47 second mark in the video below:
Clinton later said in her speech, directly referring to her electoral defeat: "No, I'm not over it. I still think about the 2016 election. I still regret the mistakes I made. [...]
"Today, as a person, I’m okay, but as an American, I'm concerned."
Clinton's jab at Trump comes as Robert Mueller's investigation into possible ties between the Trump campaign and Russia passed its first year.
Mueller has handed out various indictments and interviewed dozens of witnesses, while Trump and his allies have personally railed against Mueller and his investigation. The US president has repeatedly called the probe a "witch hunt."
RELATED: The best photos from the entire 2016 election year
The Senate Judiciary Committee recently released almost 2,000 pages of documents detailing its investigation into Donald Trump Jr.'s meeting with a Russian lawyer in June 2016 for what he hoped was damaging information on Clinton.
The New York Times also reported over the weekend that Trump Jr. met with an emissary for Saudi Arabia and the United Arab Emirates in August 2016, opening up suspicions that countries other than Russia offered to help the Trump campaign beat Clinton in the presidential election.