This paper presents the results on microbiologically influenced corrosion (MIC) of UNS S31803 duplex stainless steel (DSS) in as-received, thermally aged and welded conditions in 3.5 NaCl-based medium containing bacteria. Crevice corrosion was noticed underneath the biofilm. For as-received DSS, the corrosion was generally limited to the austenite phase, whereas in thermally aged condition a uniform dissolution of the matrix was noticed and the secondary phases were remained as skeleton. Detachment of intragranular austenite was observed in the heat affected zone (HAZ) of the weldment. The MIC behavior was found to be influenced by the microstructural changes which occurred due to the thermal treatments.