The Material Girl met the rapper, real name Limbani Kalilani, in Lilongwe.
Grin was invited to the party by his mother Health Minister, Dr Jean Kalilani, who asked Madonna if her son could attend.
Madonna is reported to have said: 'I'd like to be serenaded by a minister's son - please invite him.
The musically-minded couple spoke for a while and sang together briefly to the invited guests that included politicians, US embassy staff, diplomats and employees of Madonna's charity Raising Malawi.
The relatives of Madonna's two adopted Malawian children David and Mercy's were not on the guest list but some high school children being backed by Raising Malawi were invited.
Tay Grin, also known as 'Nyau King', scooped the 2014 Best International African gong at the Black Entertainment Film Fashion Television and Arts awards in London.
Madonna returned to Malawi today for the first time since her furious row with the country's former president 18 months ago.
The singer is believed to have flown in this morning with the two children she adopted from the central African country and her other son for surprise talks with the new leader.
Her visit will fuel speculation the star is looking to adopt another child after she posted pictures online today showing her playing with young Malawian children, apparently at an orphanage.
Madonna first visited Malawi in 2006 after her Malawian friend Victoria Keelan told her about the impoverished southern African country's Aids situation.
She immediately adopted then one-year-old David Banda at an orphanage. She adopted her second Malawian child, Mercy Chifundo James, in 2009, at another orphanage.
She periodically visits Malawi, often with all her children - including daughter Lourdes, who has not made the trip this time around.
She funds a number of orphanages, including Home of Hope in the western border district of Mchinji where she got David, and Kondanani Children's Home in the southern tea-growing district of Thyolo.
She is also building a number of schools and a paediatrics ward at the Queen Elizabeth Central Hospital (QECH) in Blantyre - which she is scheduled to visit during her trip, according to sources.