Worth knowing before you dig in: a philodendron leaf that's gone fully yellow won't turn green again. Fixing the cause protects what's left — the yellow leaf itself is worth removing rather than waiting on.
Overwatering and root rot
The most common cause on this plant, and the one to rule out first. Heartleaf philodendron likes evenly moist soil, but "moist" isn't "wet" — soil that stays soggy pushes out the air pockets roots need, and starved roots begin to rot rather than function. A plant with damaged roots can't move water to its leaves properly, which is what produces the yellowing even though the soil is full of water.
The giveaway: the leaf feels slightly soft or limp rather than crisp, the soil is visibly wet, and in more advanced cases the stem near the soil line feels mushy or the pot gives off a sour, swampy smell.
Fix: let the soil dry out fully before watering again — the top inch or two should be dry to the touch. If you suspect it's gone further, check the roots directly; healthy ones are pale and firm, rotted ones are dark and fall apart easily. Trim away anything rotted and repot into fresh, well-draining mix.
Natural aging (not actually a problem)
A philodendron that's actively vining will often let go of an older leaf near the base of the stem as it redirects energy toward new growth. The tell is that it's isolated — one leaf yellows fully while everything else, including new growth, looks fine.
Fix: nothing needed. Trim the spent leaf for a tidier look, or leave it.
Underwatering
Less common than overwatering, but it happens in warm rooms or after a long gap between waterings. The leaf tends to yellow and turn crispy or curl at the same time, rather than staying soft, and the soil itself is dry rather than damp.
Fix: water thoroughly until it runs from the drainage holes, then check the soil by touch before future waterings rather than sticking to a fixed schedule.
Nutrient deficiency
A specific and fairly recognizable pattern: the veins of the leaf stay green while the tissue between them fades to yellow, a look called chlorosis. It usually points to an iron or magnesium shortfall, often in a plant that's been in the same soil for a long time without feeding, or in soil with a pH that's locking those nutrients away from the roots.
Fix: feed with a balanced liquid fertilizer that includes micronutrients during the growing season. If the deficiency persists after regular feeding, it may be worth refreshing the soil at the next repot rather than continuing to feed into the same mix.
Too much direct light
Direct sun, especially through glass on a hot afternoon, can bleach philodendron leaves to a pale, washed-out yellow rather than the classic dry brown scorch seen on some other houseplants. It tends to be worst on whichever side faces the window and appears not long after a move into brighter conditions.
Fix: move it back from direct sun, or diffuse the light with a sheer curtain. The affected leaf won't recover its color, but new growth will be fine once the light is corrected.
Too little light
Philodendron tolerates low light reasonably well, but in genuinely dim spots it can yellow gradually as the plant can't sustain as much foliage as it's carrying, and new leaves may come in smaller and paler. This usually shows up together with longer, sparser stretches of vine and visible reaching toward whatever light is available.
Fix: move it to brighter, indirect light — a few feet from an east or north-facing window is usually enough of a step up.
Pests
Spider mites, aphids, and mealybugs feed by piercing the leaf and drawing out fluid, which can show up as yellowing, sometimes with a stippled or speckled texture rather than a solid color change. Check leaf undersides and the points 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, wipe down leaves and stems with a damp cloth, and treat with neem oil or insecticidal soap, repeating as directed since one treatment rarely clears an established infestation.