Disclosed are various embodiments for determining authentication assurance using algorithmic decay. In an embodiment, an authentication request associated with an account is received. At least one historical authentication event associated with the account is determined. A measure of authentication assurance is determined based at least in part on applying an exponential time decay to at least one authentication assurance value individually corresponding to the historical authentication event(s). A response to the authentication request is generated based at least in part on the measure of authentication assurance.