Ghazouani, 67, has been at the helm of the vast West African country since 2019 and provided stability in the Sahel region, which faces rising jihadism.
"I have deemed it appropriate to address you directly, dear compatriots, through this letter, to inform you of my decision to present myself to you, so that you renew your confidence in me for a new term," he said in a statement.
Ghazouani, the current chairman of the African Union, is the clear favorite in the polls.
Last year, his El Insaf party won a landslide victory in legislative elections— taking 107 of the 176 seats in the National Assembly, well ahead of the Islamist party Tewassoul, which won 11 seats.
The well-known anti-slavery activist and opponent Biram Dah Ould Abeid, who was a runner-up in the last presidential election, also announced his candidacy on Wednesday.
Ould Abeid, whose political group is not authorized and has no legal existence, expressed his concern about the smooth running of the electoral process.
"We are moving forward, aware of the imperfection of the electoral register and the partiality and deficiencies of the CENI (Independent National Electoral Commission," he said.
Mauritania was hit by a series of coups from 1978 to 2008, before the 2019 election marked the first transition between two elected presidents.
While jihadism has spread elsewhere in the Sahel, particularly in neighboring Mali, Mauritania has not seen an attack since 2011.
Under the terms of a presidential decree issued this month, the electoral campaign will begin at midnight on June 14 and end at midnight on June 27.
The first round will take place on June 29, with a possible second round on July 14.
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