One thing worth knowing before you dig into the cause: once a pothos leaf has gone fully yellow, it isn't going back to green. Fixing the underlying issue stops more leaves from following it, but the yellow one is worth removing rather than waiting on.
Overwatering and root rot
This is the most common cause, and the one worth ruling out first even if it isn't obvious. Soil that stays wet for too long pushes air out of the gaps between soil particles, and pothos roots — like most houseplant roots — need that air as much as they need water. Starved of oxygen, the roots stop functioning and begin to rot, and a plant with damaged roots can't move water or nutrients up to the leaves even if the soil is soaked.
The pattern that gives this one away: yellowing shows up on old and new leaves at roughly the same time, rather than working through the oldest leaves first. Stems often feel soft near the soil line, and the soil itself may smell sour or musty rather than earthy. If you're not sure, slide the plant out of its pot and look at the roots directly — healthy pothos roots are firm and pale; rotted ones are dark, mushy, and pull apart easily.
Fix: stop watering and let the soil dry out fully. If the roots are visibly rotted, trim away the dark, mushy sections with clean scissors and repot into fresh, dry, well-draining mix. Going forward, water only when the top inch or two of soil is dry to the touch, not on a fixed schedule.
Natural aging (not actually a problem)
A pothos that's producing healthy new growth will often let go of its oldest leaves — usually the lowest ones on the vine — one at a time, as it redirects energy to the new growth. This is normal turnover, not a symptom of anything wrong, and the giveaway is that it's isolated: one leaf yellows fully while the rest of the plant, including anything new, looks fine.
Fix: nothing needed. Remove the yellowed leaf once it's fully gone if you want, or leave it — either way, it doesn't indicate a care problem to correct.
Underwatering
Less common than overwatering on this particular plant, but it happens, especially in bright light or a heated room in winter. A pothos running short on water will often yellow and curl at the same time, and the leaves tend to feel dry and slightly crispy rather than soft. Vines may also look limp or droopy before the yellowing sets in.
Fix: check the soil — if it's pulling away from the sides of the pot or feels bone-dry more than an inch down, water thoroughly until it drains from the bottom, then go back to checking by touch rather than a rigid schedule.
Too little light
Pothos tolerates low light better than almost any houseplant, but "tolerates" isn't the same as "thrives." In genuinely dim conditions, new leaves can come in pale or washed-out, and older leaves may yellow gradually as the plant simply doesn't have the energy to maintain as much foliage as it's carrying. This tends to show up alongside long, sparse stretches of vine with more space between leaves than usual.
Fix: move it to a spot with more indirect light — a few feet from an east or north-facing window is usually enough of a step up without risking scorch.
Too much direct light
The opposite problem looks different: yellowing that comes with dry, scorched brown patches, usually concentrated on the side of the plant facing the window, and appearing not long after a move into brighter conditions. This is heat and UV damage, not a nutrient or water issue.
Fix: move it back from direct sun, or add a sheer curtain to diffuse the light. Reintroduce brighter light gradually if you want it in a sunnier spot eventually.
Nutrient deficiency
Less common than the watering and light causes above, but worth knowing the pattern: nitrogen is a mobile nutrient, meaning the plant can pull it out of older leaves and redirect it to new growth when the soil supply runs low. That produces fairly uniform yellowing across whole older leaves while new growth at the tips stays green — a different pattern from the aging case above, since it tends to affect more than one leaf at a time and progresses steadily rather than one leaf fully going before the next starts.
Fix: resume feeding with a balanced liquid houseplant fertilizer at half strength during the growing season — but only after confirming the soil isn't waterlogged and the roots are healthy, since fertilizing a plant with root damage doesn't help and can make things worse.
Pests
Spider mites, scale, and mealybugs all feed by piercing the leaf and drawing out fluid, which shows up as yellowing, sometimes with a stippled or speckled texture rather than a solid color change. Check the undersides of leaves and the tight junctions where leaves meet the stem — fine webbing points to spider mites, small bumps or cottony fluff point to scale or mealybugs.
Fix: isolate the plant from others, wipe down leaves and stems with a damp cloth, and treat with insecticidal soap or neem oil, repeating as directed since a single treatment rarely clears an established infestation.
Fertilizer burn
Too much fertilizer, or fertilizing too often, builds up mineral salts in the soil that can scorch roots and show up as yellowing combined with brown, crisp leaf margins or a visible white or yellowish crust on the soil surface. This tends to follow a recent feeding rather than appearing gradually.
Fix: flush the soil thoroughly with plain water — several times the volume of the pot — to wash out excess salts, and cut back on how often you feed going forward.
Transplant stress
Some yellowing shortly after repotting is common, particularly if the roots were disturbed or the plant moved into soil that holds moisture differently than before. It's usually temporary.
Fix: keep light and watering stable and skip fertilizing for a few weeks while the roots settle in. Avoid the temptation to move it again or change anything else at the same time — one variable at a time makes it possible to tell what actually helped.