In this study, low-carbon steel plates (ASTM 131A) used generally in shipbuilding applications were joined by friction stir welding (FSW) using optimum processing parameters. The microstructure, mechanical properties, formability, and corrosion behavior of the joint were investigated. From the results of the investigation, it was discovered that two distinct regions, stir zone (SZ) and heat-affected zone (HAZ), were formed in the welded region during FSW of the plates. FSW decreased the ferritic grain size of the SZ from 25 mu m to about 4 mu m. Refined grains were separated mostly by a high angle of misorientation with the low amount of dislocations. The hardness of the SZ increased from 140 Hv0.3 to about 230 Hv0.3. The yield and tensile strength values of the SZ increased from 256 and 435 MPa to about 457 and 585 MPa, respectively, by the effect of FSW without a considerable decrease in ductility. FSW did not cause a significant change in the formability of the joint. Displacement at the maximum bending force decreased slightly after FSW from 6.31 mm to about 5.69 mm. Also, corrosion resistance after FSW slightly increased as a result of grain refinement. Current density values were obtained as 3.36 x 10(-6) A/cm(2) (base material 4.44 x 10(-6) A/cm(2)).