In 2015, the North African country was the target of several attacks claimed by the Islamic State group that killed 72 people.
"A Salafist attacked two policemen with a knife. One was struck on the forehead, and the other stabbed in the neck and is in intensive care," ministry spokesman Yasser Mesbah said.
The assailant was arrested and confessed to having adopted three years ago an extremist line of thought that views members of the security forces as "tyrants", the ministry said in a statement.
"Killing them, he believes, is a form of jihad," it said.
Walid Hkima, a spokesman for the national security forces, told state television the attack happened at around 8am (0700 GMT) and the assailant was quickly arrested.
Since its 2011 revolution, which sparked the Arab Spring, Tunisia has faced a series of jihadist attacks that have claimed the lives of more than 100 soldiers and police.
In 2015, the North African country was the target of several attacks claimed by the Islamic State group that killed 72 people -- 59 of them foreign tourists.
The country has been under a state of emergency since November 2015, after one of those attacks killed 12 presidential guards in the heart of the capital.