Some of the Bulgarian capital's main streets will be closed for the duration of McAleese's stay as a security measure.
Her visit comes at a time when Irish-Bulgarian relations are very much in the news ahead of Saturday's crucial football World Cup qualifer between the two countries in Ireland.
McAleese is scheduled to meet Bulgarian President Georgi Parvanov as well as Georgi Pirinski, Speaker of the National Assembly and Bulgarian PM Sergey Stanishev.
She will also spend one day of her visit in Bulgaria's second largest city of Plovdiv, before being the guest of honor at a reception for the Irish community in Bulgaria.
She will leave Bulgaria on Friday, and will attend the World Cup match on Saturday in Dublin.
Mary McAleese was first elected president in 1997 and won a second term, without a contest, in 2004. McAleese was born in Belfast, becoming the first Irish President to come from Northern Ireland. Prior to becoming president she was a barrister, journalist and academic.