Is it illegal to run Romex in conduit?

Is it illegal to run Romex in conduit, no, it is not illegal to run Romex NM-B cable in conduit in many situations, but it is restricted and must follow code, with NEC allowing Romex in conduit for protection where required, but not in wet locations and with fill and derating rules. NEC allows NM cable to be run in conduit for short sections where protection from physical damage needed, such as where Romex passes through concrete, or in unfinished basement to protect, with conduit providing protection. However, you cannot run Romex in conduit outdoors or underground where conduit is considered wet location, because Romex is not rated for wet locations, so illegal there, must use UF or THWN. Also conduit fill limits apply, as Romex is flat and fill calculation must consider, and derating may apply if many cables in conduit causing overheating. So is it illegal to run Romex in conduit, no, not illegal if done per code for protection indoors in dry locations, with short runs allowed, but illegal in wet locations outdoors and underground, and must follow support and fill rules, with many electricians preferring THHN individual wires in conduit rather than Romex because easier to pull and more code compliant, with local inspector may have interpretations, so check local code, but generally allowed with conditions, not blanket illegal.