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PG 1553+113 is a well-known blazar exhibiting evidence of a $\sim\! 2.2$-yearquasi-periodic oscillation in radio, optical, X-ray, and $\gamma$-ray bands. Wepresent evidence of a new, longer oscillation of $21.8 \pm 4.7$ years in itshistorical optical light curve covering 100 years of observation. On its own,this $\sim\! 22$-year period has a statistical significance of $1.9\sigma$ whenaccounting for the look-elsewhere effect. However, the probability of both the$2.2$- and $22$-year periods arising from noise is $\sim0.02\%$ ($3.5\sigma$).The next peak of the 22-year oscillation should occur around July 2025. We findthat the $\sim\,$10:1 relation between these two periods can arise in aplausible supermassive black hole binary model. Our interpretation of PG1553+113's two periods suggests that the binary engine has a mass ratio$\gtrsim 0.2$, an eccentricity $\lesssim 0.1$, and accretes from a disk withcharacteristic aspect ratio $\sim 0.03$. The putative supermassive black holebinary radiates nHz gravitational waves, but the amplitude is $\sim10-100$times too low for detection by foreseeable pulsar timing arrays.

Constraining the PG 1553+113 binary hypothesis: interpreting a new, 22-year period