A sheet thickness detector incorporated in an image forming apparatus includes a sheet conveying member to rotate and convey a sheet in a sheet conveyance direction, a driven sheet conveying member to contact the sheet conveying member and form at least one first transfer nip therebetween in a lateral direction and to displace by an amount equivalent to a thickness of the sheet passing through the first transfer nip and rotated with the sheet conveying member in the sheet conveyance direction, a displacement member to contact the sheet conveying member and form a second transfer nip smaller than the first transfer nip in the lateral direction and to displace by an amount equivalent to the thickness of the sheet passing through the second transfer nip and supported at a support member, and a displacement amount detector to detect an amount of displacement of the displacement member.