Tonsillectomy is commonly performed to treat repetitive tonsillitis. Tonsillectomy changes the shape and characteristics of the pharynx. It could also alter the resonating characteristics of the pharynx and affect the voice of the patient. We examined voice changes in patients undergoing tonsillectomy. A total of 33 adult patients who underwent tonsillectomy at our institution between July 2020 and December 2021 were included in the study. The participants consisted of 16 men and 17 women, with a mean ± standard deviation age of 33.7 ± 10.7 years. We measured the maximum phonation time, acoustic analysis values (jitter%, shimmer%, and noise-to-harmonic ratio [NHR]), speaking fundamental frequency and vocal range, first four formants (F1–F4) in the sustained vowel /a/, and Voice Handicap Index (VHI). We examined whether any changes occurring 3 months after the surgery compared with the findings before the surgery. The shimmer% and NHR decreased significantly after surgery compared with the observations prior to surgery. Among women, the speaking fundamental frequency tended to increase, and the F2 significantly increased after the surgery compared with that prior to surgery. Removal of the swollen tonsils reduced the resistance of the supraglottic space and resulted in a lower laryngeal adduction force required for phonation. The decreases in the shimmer% and NHR were attributed to a reduction in the turbulence in the supraglottic space. The change in the shape of the pharynx caused changes in the speaking fundamental frequency and formants. The changes were thought to be influenced by the size of the patient's pharyngeal cavity, volume of the tonsils, and scarring of the pharyngeal mucosa. Patients should be informed before surgery of the possibility of voice changes.