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abstract: 'We study the long-term variability in the optical monitoring database of Ark 120, a nearby radio-quiet active galactic nucleus (AGN) at a distance of 143 Mpc ($z=0.03271$). We compiled the historical archival photometric and spectroscopic data since 1974 and conducted a new two-year monitoring campaign in 2015-2017, resulting in a total temporal baseline over four decades. The long-term variations in the optical continuum exhibit a wave-like pattern and the H$\beta$ integrated flux series varies with a similar behaviour. The broad H$\beta$ profiles have asymmetric double peaks, which change strongly with time and tend to merge into a single peak during some epochs. The period in the optical continuum determined from various period-search methods is about $20$ years and the estimated false alarm probability with null hypothesis simulations is about $1\times10^{-3}$. [The overall variations of the broad H$\beta$ profiles also follow the same period. However, ]{} the present database only covers two cycles of the [suggested]{} period[, which strongly encourages]{} continued monitoring to track more cycles and confirm the periodicity. Nevertheless, in light of the possible periodicity and the complicated H$\beta$ profile, Ark 120 is one candidate of the nearest radio-quiet AGNs with possible periodic variability and [it is 