The suit to be unveiled Monday centers on the so-called emoluments clause, which bans US officials from taking gifts or benefits from foreign governments.
Since taking power in January, Trump has turned day-to-day control over his real estate empire and other assets to his adult sons, but not sold them off as many in America said he should to avoid conflicts of interest.
Maryland Attorney General Brian Frosh said the case is about Trump's failure to separate his personal interests from his presidential duties.
The suit will seek an injunction to force Trump to stop violating the Constitution, but will leave it up to the court to decide how that should be accomplished.
