The San Diego-based amphibious assault ship USS Makin Island transited the narrow Strait of Hormuz and entered the Persian Gulf on Monday, the Navy reported.
The Makin Island was accompanied by the guided-missile cruiser USS Port Royal and dry cargo ship USNS Carl Brashear.
Other ships in the Makin Island Amphibious Ready Group, including the amphibious transport dock ships USS Somerset and USS San Diego, entered the gulf a number of days earlier.
The ready group includes Marines from the 15th Expeditionary Unit based at Camp Pendleton.
While in the Gulf, the 5,000 sailors and Marines of the Makin Island group will train alongside regional allies and provide naval aviation support to Operations Inherent Resolve and Enduring Freedom.
The Makin Island is similar to a small aircraft carrier and supports more than 30 helicopters, Osprey tiltrotors and F-35B stealth jets.