In most uses, "niche" is a noun. Nouns refer to a person, place, or thing. A niche is a thing.

However, there are occasions when "niche" gets used as an adjective. In the phrase "niche marketing," "niche" modifies :marketing," so it's an adjective in that usage.

Robert B. Settle has written:

'Niche Marketing, How to Pinpoint Target Markets'

'Niche Marketing'

Niche marketing means a specific term that matches that market your business is involved in. It is more of a direct marketing platform instead of using the shotgun method.

What defines the members of a niche market is not the market size, but some unique characteristic shared by all members of that niche market. Marketers then use that to inform decision-making about marketing strategy and develop tactics to understand the consumer needs of that group then create marketing plans to target that niche group. Niche marketing is the resulting tactic of implementing marketing strategies specifically for targeting a niche market segment, often by capitalizing on the presence of a unique characteristic of that market. For example, the market for people aged over 55 is one example of a 'niche market'. Similarly the African-American market is a 'niche' market, even though it is a large group of consumers. The gay market is another niche market group of millions of consumers around the world. So-called 'grey' marketing, African- American marketing and gay marketing are examples of the resulting marketing strategies that result from defining segments by way of niche market approaches. Niche marketing has been shown to be a very effective means for mass market brands to seek to better position their products and services to appeal to niche market consumers.

a company with limited resources might decide to serve only one or a few special segment