Short answer
Yellowing leaves on a ZZ plant are almost always caused by overwatering, which leads to rot in the specialized underground rhizomes. The most effective fix is to immediately withhold water, check the rhizomes for mushiness, and ensure the soil is allowed to dry out completely between waterings.
Zamioculcas zamiifolia is incredibly hardy, so yellowing is a notable signal that something in its environment—usually soil moisture—is off balance. While older leaves naturally yellow and drop as part of the plant's life cycle, widespread or sudden yellowing suggests you should inspect the plant's health immediately.
The ZZ plant stores water in thick, tuberous rhizomes that are highly susceptible to rot if the soil remains constantly damp. Excessive moisture prevents these storage organs from oxygenating, causing the stems to yellow, soften, and eventually collapse.
Check: Gently dig down near the base of a yellowing stem; if the rhizome feels mushy or smells foul rather than firm and white/tan, it is rotting.
Fix: Cease watering immediately. If rot is detected, remove the plant from the pot, trim away all mushy, black roots and rhizomes with sterilized shears, and repot in fresh, well-draining cactus or succulent soil.
As the ZZ plant matures, it is normal for the oldest, lowest leaves to gradually turn yellow and eventually wither as the plant reallocates energy to new growth. This process is slow and usually affects only one or two leaflets at a time.
Check: Check that the rest of the plant remains deep green and firm, and that there are no signs of mushy stems or unusual spots.
Fix: No action is required; you can simply snip off the yellowed leaf once it has completely turned brown and brittle.
While ZZ plants are known for their tolerance of low light, extremely dark corners can eventually stress the plant to the point of pale or yellowing foliage. If the plant is in a windowless room, it will eventually struggle to maintain its deep green color.
Check: Check if the yellowing is accompanied by 'leggy' or stretched-out stems reaching for a light source.
Fix: Move the plant to a spot with brighter, indirect light, such as near a window with a sheer curtain, to help it regain its vigor.
When to worry
You should worry immediately if you notice a cluster of yellow leaves and the stems feel soft at the soil line, as this indicates active rhizome rot that can kill the entire plant if not addressed.
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Diagnose my ZZ PlantIt is best to wait until the leaves have completely yellowed or turned brown before removing them. Pruning while they are still partially green can interfere with the plant's natural process of pulling back nutrients.
If you only watered it once, you might just let it sit and dry out for several weeks without adding more water. However, if the soil is staying wet for more than a few days, repotting into dry, fresh soil is a safer way to prevent rot.
While there are plant viruses, they are extremely rare in indoor ZZ plants and are not a standard concern for hobbyists. Focus on correcting your watering and light levels first, as these solve 99% of ZZ plant issues.
Consistent with guidance from: Royal Horticultural Society, University of Florida IFAS Extension.