Background: Mpox virus (MPXV) clade Ib, first detected in the Democratic Republic of the Congo (DRC) in September 2023, spread internationally within months, prompting an emergency declaration from the World Health Organization. Data on its incubation period, which both shapes outbreak dynamics and informs epidemic response strategies, remain limited.

Objective: To estimate the incubation periods of mpox clade Ib, examining evidence for differences by route of exposure and demographic factors.

Design: Bayesian analysis of clinical surveillance data collected between June and October 2024.

Setting: South Kivu, DRC, the epicenter of the current mpox clade Ib global outbreak.

Participants: Clinically attended persons with confirmed mpox clade Ib infection.

Measurements: Demographic characteristics, exposure history, symptom onset, and transmission route.

Results: Among 37 polymerase chain reaction-confirmed cases with high viral load (cycle threshold values <34), the median incubation period from exposure to rash was 13.6 days (95% credible interval [CrI], 9.6 to 19.0 days). Five percent of cases are expected to develop a rash within 3.1 days (CrI, 1.3 to 5.5 days) and 95% within 32.3 days (CrI, 22.4 to 45.8 days). The incubation period seemed to differ by putative transmission route: Sexual transmission had a shorter median (10.3 days [CrI, 3.1 to 20.3 days]) than nonsexual transmission (13.5 days [CrI, 9.5 to 19.1 days]), although the CrIs overlapped.

Limitation: Surveillance data lacked detailed exposure histories and a lower bound for exposure periods, but models accounted for these uncertainties, yielding robust median estimates.

Conclusion: Evidence from this study suggests that clade Ib may have a longer incubation period than other MPXV clades, and this may vary by transmission route. The shorter incubation for sexual transmission mirrors patterns seen in the predominantly sexually transmitted clade IIb outbreak, highlighting the potential role of exposure route in disease progression. These findings have implications for global recommendations on postexposure monitoring periods and prophylaxis.