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. He can get quite tired after working during the night, so he sometimes has an embrassing habbit of occasionally falling asleep in mid-sentence. 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 (cameo), Foduck Stays Home, Theodore, The Tug In Charge, and Theodore's Day Off (cameo)
- Season 3 - George and the Underwater Mystery, Theodore's Big Friend, Theodore and the Borrowed Bell, Emily's Close Call, Theodore and the Pirate and Big Harbour Fools Day (does not speak)
- Season 4 - Theodore's Tough Tugging, Hank Makes a Friend, Northumberland is Missing, George and the Navy Ship, and Owan On the Loose
- Season 5 - Theodore and the Harbour Crane (does not speak), Hank and the Sunken Ship, Hank Stays Up Late, Theodore the Tattletug, Theodore and the Hunt for Northumberland, Guysborough Makes a Friend, Bedford's Big Move (mentioned), Emily and the Sleep-Over, 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.
- In the Finnish version, Northumberland is called Huhmeri.
- For the underwater scenes, a small scale version of Northumberland hung by wire is used to show him moving through the water while his regular scale version is only used for close-up shots.
- ERTL (die-cast and bath toy; discontinued)