Visitors to Senegal require a visa unless they come from one of the visa exempt countries.
Visa exemption
Citizens of the following 125 countries can visit Senegal without a visa for up to 90 days:
Nationals of Tanzania can obtain a visa on arrival if have an official letter of invitation and a return/onward ticket.
Holders of diplomatic passports issued to nationals of Brazil, China, Iran, Turkey and Uganda and holders of diplomatic and official/service passports issued to nationals of Algeria, Gabon, Libya, Russia and Uganda do not require a visa.
Holders of an official invitation letter do not require a visa.
References
Visa policy of Senegal Wikipedia(Text) CC BY-SA