Trump has adamantly denied any collusion with Russia as well as any business associations in the country. Trump's claim he has no real estate holdings in Russia was recently contradicted by witnesses testifying before Mueller.
Nagging questions about Moscow trip
Mueller is also scrutinizing Trump's 2013 trip to Moscow to host the Miss Universe pageant and the $20 million (€16.3 million) fee he collected for doing so.
At the time, Trump claimed to have met Russian President Vladimir Putin, praising his "amazing" leadership and bragging: "I do have a relationship with him." Since then, Trump has repeatedly denied the two ever met.
The subpoena is the first indication that the special counsel is methodically approaching the issue of the president's business dealings, something Trump has previously said would be a "red line" for him. The president did not, however, say what would happen should the special counsel cross that line.
Not wrapping up anytime soon
Mueller was appointed to investigate the possible role that Russia played in the 2016 US presidential election in May 2017. His appointment was made by Deputy Attorney General Rod Rosenstein after Attorney General Jeff Sessions recused himself and Trump fired then FBI Director James Comey.
Last week, the Republican-led House Intelligence Committee ended its investigation into the 2016 election after it found no evidence of collusion between the Trump administration or campaign team and Russia. Trump claimed he was vindicated, but Democrats on the committee complained the decision was premature.