The US Department of State says yes, but as far as the Rwandan government is concerned, the US Department of State is a secondary source. Oddly, they say
Yellow fever vaccination required upon entry
This is odd because the vaccination requires 10 days before it is effective. Source: http://travel.state.gov/content/passports/en/country/rwanda.html
TIMATIC says no:
Rwanda (RW) Vaccinations not required. Recommended: - Vaccination against yellow fever for all passengers over 1 year of age.
(Retrieved via klm.com)
But what does the Rwandan government say? The best I could find, via the Directorate General of Immigration and Emigration, is
A vaccination certificate may be required at the border post. In the event of a major disease outbreak, specific requirements could be set and put to the knowledge of the public.
(emphasis added; source: https://www.migration.gov.rw/index.php?id=13)
Is there any authoritative (and unambiguous) source indicating the requirements for entering Rwanda as a tourist?