Northumberland is a black submarine who sleeps during the day and works during the night.
He is hardly ever awake in front of the tugs, except for when Theodore and Hank see him on their night shift. Northumberland is a very quiet sub and hates loud noise and bright lights. He also helps the "Salvage Team" (Theodore, Rebecca and Shelburne) to find great things under the water. He never likes to disappoint his friends, which can sometimes get him or his friends into danger. He once broke his propeller in an underwater rock slide and showed Hank where a sunken ship was. Northumberland is also a big brother-like figure to Pugwash, but was once scared that she would bump him. Northumberland is always a great help to the tugs and the Salvage Crew and whether he's awake or half-awake, he is always willing to be some assistance to anyone in the Big Harbor. Northumberland lives at the Navy Yards and helps Foduck with his safety duties, but he is always welcomed anywhere.
Northumberland is loosely based off a Canadian Naval submarine.
Northumberland is painted black with his name on both sides painted in white.
- Season 1 - Theodore and the Welcome
- Season 2 - Night Shift, The Tugboat Pledge, Foduck Stays Home, Theodore, The Tug In Charge, Theodore's Day Off (cameo), and Theodore's Day Off
- Season 3 - George and the Underwater Mystery, Theodore and the Missing Barge, Theodore and the Borrowed Bell, and Big Harbour Fools Day (does not speak)
- Season 4 - Theodore's Tough Tugging, Northumberland is Missing, George and the Navy Ship, and Owan On the Loose
- Season 5 - Theodore and the Harbour Crane (does not speak), Theodore and the Hunt for Northumberland, Emily and the Sleep-Over, Hank and the Sunken Ship, Foduck and the Shy Ship, and Theodore's Big Decision
- He was named after the Northumberland Strait, the body of water that devides New Brunswick and Nova Scotia on the mainland of Canada from Prince Edward Island.
- ERTL (die-cast and bath toy; discontinued)