Aziz, a 66-year-old former general, appeared in a court in Nouakchott among 10 defendants, including former prime ministers, ministers and businessmen, AFP reports.
They face charges that include abuse of office, influence peddling, money laundering and illicit enrichment.
Aziz is suspected of siphoning money from state contracts or the sale of real estate, amassing a fortune equivalent to more than $72 million.
He has denied the allegations against him but refused to answer investigators' questions about the source of his wealth.
He describes himself as the victim of score-settling and argues he has immunity from prosecution under the constitution.
"He completely rejects the accusations against him," one of his lawyers, Taleb Khayar Ould Med Mouloud, said ahead of the trial.
"Lots of people have grounds for hating him, including the Muslim Brothers, which he threw out of the country."
Another of his lawyers, Antoine Vey, expressed alarm at his client's "arbitrary" arrest and conditions that he said suggested Aziz would not be given a fair trial.
"The case has been built on work that just looks like political revisionism," he told AFP.
He planned to ask for a postponement of the trial and was preparing to refer the case to United Nations bodies if the request is rejected.
Brahim Ebetty, one of several lawyers representing the state, told AFP "It is a first in the history of Mauritania and perhaps even in the Arab world for a former president to explain his enrichment."
"All the people in the box [dock] have used the name of the state, the function of the state, (to enrich themselves) -- especially Mr Aziz," Ebetty said.
The former president has denied the allegations but refused to answer investigators' questions about the sources of his wealth.
Aziz, a trader's son who came to power in a coup, stepped down in 2019 after two presidential terms in which he defused a jihadist insurgency threatening the conservative West African state.
He was succeeded by his former right-hand man, Mohamed Ould Ghazouani, in the first transfer of power between elected leaders in the history of a country marked by military coups and upheaval.
But within month of the handover, allegations emerged of financial mis-dealings.
Aziz is suspected of siphoning money from state contracts or the sale of real estate, amassing a fortune equivalent to more than $72 million.
He denies the allegations against him and says he is the victim of a settling of scores.
Mauritanians interviewed by AFP hope the trial will set a new benchmark in the fight against graft in their country.
Mauritania is ranked a lowly 140th out of 180 in the 2021 Corruption Perceptions Index by the organization Transparency International.