Garamendi resigned the post after being elected to Congress from California's 12th District in a special election this month.
Schwarzenegger announced the appointment of Maldonado during an appearance Monday on "The Jay Leno Show."
"(Sen. Maldonado)'s a terrific, loyal man that has worked very hard in public service," Schwarzenegger said. "But he's also into bipartisanship and post-partisanship, so he can reach cross the aisle."
Maldonado is seen as a moderate who has bucked Republican leaders in Sacramento on key votes at Schwarzenegger's request, the Los Angeles Times said. The appointment must be confirmed by both houses of the California Legislature before it can take effect.
Schwarzenegger also joked on the show he will take over "The Oprah Winfrey Show" when he leaves office. Winfrey announced last week she will end her long-running TV talk show in 2011.