Officials from President Recep Tayyip Erdogan's office said the pledge was made by Sheikh Tamim bin Hamad Al Thani, who met with the Turkish leader Wednesday.
The officials provided the information only on condition of anonymity, in line with government rules. They did not provide further information on the nature of the investments.
During the meeting, Erdogan and Al Thani also expressed their determination to develop ties between their countries "in all areas," according to the officials.
Turkey and Qatar have forged close bonds over the years, with Ankara supporting Doha during its standoff last year with other Gulf neighbors, including sending troops to a Turkish base there.