Adults
- Otis Weaselgraft: One of the antagonists of the TV series, who wants to dognap Martha so he can make money off her special ability, and calls Martha "talking dog", but is constantly thwarted by Martha and her young friends. His main downfall is he underestimates Martha's intelligence. In one episode, he sold T.D. a "steak tree" as a scam, but even though the main characters discovered it didn't grow steaks, he wasn't caught. He and Pablum are recurring antagonists in the series, despite repeatedly getting caught and defeated in his scams by Martha. Both he and Pablum always manage to escape arrest.
- Dr. Harold Pablum: An antagonist of the TV series, Pablum is a scientist who serves as Otis Weaselgraft's sidekick/minion. Despite trying his best to help Otis, he is constantly abused by him.
- Granny Florence "Flo": Owner of the factory which makes the Alphabet Soup Martha needs to speak. She is shown to be greedy and regrets having to put "every letter in every can" (per the company's slogan).
- Granny Elsie: The founder of Martha's alphabet soup.
- Alfred "Al" Abbott: Voiced by Samuel Vincent. He is the alphabetician who makes the letter A for the alphabet soup Martha eats. He is laid off when Granny Flo decides to cut costs by getting rid of half the letters for the soup, but gets it back after he shares his last can of complete alphabet soup with Martha, who scares Granny Flo into giving everyone back their jobs.
- Kazuo: Initially voiced by Matt Hill for the first season, and later replaced by a different, uncredited actor. The dog pound owner. He wants to make sure all the dogs at the pound get good homes and ends up adopting Pops, one of the longtime residents at the pound. While working,he wears earmuffs with a radio.
- Omar Gaelic Kennelly: T.D.'s dad. He owns a workshop and is shown to be quite eccentric and enjoys inventing things. He is voiced by Terry Klassen.
- Janice Kennelly: T.D.'s mother. She is the vice-principal of the school.
- Mrs. Eulah Demson: Helen's neighbor and the neighborhood grouch. She is often grumpy, selfish, conniving, and often intolerant of others' activities. She can barely stand Martha, however, it has been shown that she secretly likes Martha. She has loud alarms on her front porch that go off when someone rings her doorbell. She has a young granddaughter named Felicity that will appear in season four who is a mute and is very timid of Mrs. Demson she resembles and acts like Ms. Finster from Disney's Recess.
- CK: TD's uncle. He's a farmer who Helen, Timmy. David and Martha once visited. His farm is seen in the episode "Down on the Farm" and was mentioned in the episode "Skits Behaves".
- Louie Kablooie and James "Jimmy Gimmie" Moore: Two crooks who attempted to rob Aprin Jewellery Store in Codename: Martha, but were foiled by Martha and, according to T.D., sent to jail. Codename: Martha was the only time they ever appeared.
- Karl: The man who owns the butcher shop.
- Mrs. Clusky: Voiced by Nicole Oliver, she is Helen's teacher. She is shocked when she finds out Martha can speak, though she has since warmed up to it.
- Cisco: A Russian old man.
- Lucy "Lucille" Lorraine: Daniel's mom. She doesn't like bacon, and she is allergic to flowers.
- Bernard "Bernie" Lorraine: Daniel's father. He once lost Lucille's keys, bended her skies, and sold Lucille's chair for a big grizzly bear.
- Boomer: An old man with fading sight, often losing his glasses.
- Mrs. Parkington: An old lady who believes in the ghost of her Great Aunt Martha.
- Windy McCloud: Wagstaff City's meteorologist.
- Mrs. Oatley: Truman's mom. She owns a day care center run at home.
- Mr. Oatley: Truman's dad.
- Brunhilde: An opera singing woman best for singing high notes.
- Terrence Lee: Milo's father.
Read more about this topic: List Of Martha Speaks Characters, Secondary Characters
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