Postoperative outcome of diffractive multifocal intraocular lens (MIOL) implantation in eyes with a small-diameter pupil was evaluated. This non randomized case series involved 23 eyes of 23 patients who underwent diffractive MIOL implantation and whose preoperative photopic pupil diameter was ≤3.0 mm [small-diameter pupil (SDP) group], and 79 eyes of 79 patients implanted with the same MIOL whose pupil diameter was >3.0 mm as controls (LDP group). Contrast sensitivity of high spatial frequency (12, and 18 cycle per degree) and both 12.5% and 6% low-contrast visual acuity (VA) were significantly worse in the SDP group than in the LDP group (P = 0.04, 0.05, 0.05, 0.03). However, no significant difference was found between eyes in the SDP group with a postoperative pupil diameter greater than 3.0 mm and LDP group eyes. No significant differences were found in uncorrected and corrected distance VA (UDVA, CDVA), uncorrected and corrected near VA (UNVA, CNVA), contrast sensitivity with low spatial frequency, and defocus curve between the two groups. In conclusion, in eyes implanted with a diffractive MIOL, a pupil diameter of ≤3.0 mm deteriorates contrast sensitivity. This effect was eliminated postoperatively when pupil size was enlarged to >3.0 mm during surgery.