The deployment was requested by the Pentagon to support the fight against militant group al-Shabab.
President Trump withdrew about 700 US troops from Somalia in 2020.
The move to re-establish a military presence in the East African country comes as long-overdue elections delivered a new president.
Hassan Sheikh Mohamud, a former peace activist, promised to work closely with international partners as he took office on Monday.
Somalia has suffered from decades of chronic insecurity, and the Islamist militants who once controlled the country still hold large swathes of it and continue to collect taxes in places.
Many in the country expressed deep concern when former president Donald Trump ordered the withdrawal of US troops, who had long been relied on as well as more than 19,000 peacekeepers from African Union nations.
This time around, fewer than 500 US troops will be deployed, which has been described as "a repositioning of forces already in theatre who have travelled in and out of Somalia on an episodic basis" by US National Security spokeswoman Adrienne Watson.
Her statement appeared critical of the Trump administration, calling its decision to withdraw troops "precipitous".
Al-Shabab militants regularly carry out attacks in the capital Mogadishu, which they stepped up in the run-up to May's election in the hopes of derailing it.
Somalia faces other formidable challenges including a drought that has left millions in urgent need of aid.
