Corruption has become more entrenched in Burundi, blamed on among other things weak political leadership to fight against it.
According to the Transparency International’s annual Corruption Perceptions Index, the country slid five points, moving from 22 in 2017 to 17 in 2018.
With the exemption of Rwanda, other East African countries do not fare any better.
For the seventh year in a row, Somalia and South Sudan, both ravaged by years of conflict, remain the most corrupt countries in the world scoring 10 and 13 points respectively.
TI cited few political rights, limited press freedoms and a weak rule of law as some of the factors for rampant corruption.
"Laws often go unenforced and institutions are poorly resourced with little ability to handle corruption complaints. In addition, internal conflict and unstable governance structures contribute to high rates of corruption," TI said in a statement on Tuesday.
The TI scale runs from 0 to 100, where zero means very corrupt and 100 very clean.
Rwanda with 56 points is the least corrupt country in East Africa, and fourth in Africa after Seychelles, Botswana, and Cape Verde.
Observers credit Rwanda's performance to President Paul Kagame’s strict enforcement of compliance with the country's leadership code
Since coming into power in November 2015, Tanzanian President John Magufuli anti-corruption drive saw the country make significant progress but it seems to be losing steam having scored 36 in 2018, same as it did in 2017.
Uganda did not make any improvement as well, scoring 26.
Kenya dropped by a point to 27 from 28 in 2017 but TI placed it under its watchlist of sub-Saharan African countries -- others are Angola, Nigeria, Botswana, and South Africa -- with new anti-corruption commitments.
"The real test will be whether these new administrations will follow through."
In ranking terms, Rwanda, the only country in the region that fares well in anti-corruption ratings, retained its position at number 48.
Burundi dropped 13 positions to rank 170 out of 180, from 157.
Kenya dropped by one position to 144.
Tanzania rose four positions to 99 from 103 in 2017, with Uganda improving from 151 to 149.
South Sudan was position 178 while Somalia trailed at 180.