Yes. Moss is a genuine plant, every bit as much as an oak or a daisy, but it belongs to a very old and very plain branch of the plant kingdom called the bryophytes, one that has got by for hundreds of millions of years without most of the equipment a garden flower depends on. It is green, it makes its own food from sunlight, and it grows, reproduces and dies as a plant does. What throws people is everything it does without, not any doubt about what it is.
Why it counts as a plant
The test is simple. A plant is a living thing that holds its own chlorophyll and feeds itself by photosynthesis, turning light, water and air into sugar. Moss does exactly that: the green of a cushion is chlorophyll at work, which is why moss needs light and grows in the damp shade rather than the dark. That alone separates it cleanly from a fungus, which has no chlorophyll and must live off other things. Moss is one of the bryophytes, the group of small, non-vascular plants that were among the very first to leave the water and settle on dry land, long before ferns, conifers or anything that has ever borne a flower.
What sets it apart from other plants
Moss is a plant that does without a long list of things. It grows no true roots, only fine threads called rhizoids that grip a surface without feeding the plant, set out under do mosses have roots. It has no internal plumbing, none of the tubes a larger plant uses to lift water up a stem, which is why it is called non-vascular and why it stays low and drinks rain straight off its leaves. It never flowers and sets no seed; instead it scatters dust-fine spores from a capsule lifted on a slender stalk. None of these absences make it any less a plant. They simply make it a plainer, older kind than the flowering plants most of us picture first.
The things mistaken for moss that are not plants
Plenty that carries the name "moss" is not a plant at all. Lichen, the crusty grey and yellow growth on gravestones and bark, is a partnership of a fungus and an alga, and reindeer "moss" is one of these lichens rather than a true moss. Spanish moss, by contrast, is a flowering plant related to the pineapple, so it is a plant but not a moss. Sorting the real article from this crowd is the business of telling moss apart from its lookalikes.
Common questions
Is moss a plant or a fungus?
Moss is a plant, not a fungus. It is green and makes its own food by photosynthesis, which fungi cannot do. The confusion usually comes from lichen, the crusty growth on gravestones and twigs, which is part fungus and often mistaken for moss.
Why is moss called a non-vascular plant?
Because it never grew the internal tubes, the xylem and phloem, that larger plants use to carry water and food from root to leaf. Lacking that plumbing, moss stays low and drinks rain straight off its leaves, which is what botanists mean by non-vascular.
Is moss a plant if it has no roots?
Yes. True roots are not what makes something a plant. Moss anchors with fine threads called rhizoids and takes water and minerals in across its whole surface, so it manages without roots while remaining every bit a plant.
For the plain sketch of what moss is, see what is moss, and for the workings in full, the biology of mosses sets out the two-stage life cycle and how the plant is built.