A Licensed Master Social Worker is an American credential held by social workers who have completed a master's degree in social work and passed a licensing examination, but who have not yet accrued the supervised clinical hours required to move on to independent practice at the LCSW level.
An intermediate stage, not a separate profession
LMSWs work across mental health, healthcare, schools and community settings, providing clinical services under supervision throughout this stage of their career. It is important to understand this credential as a stage on the route toward the LCSW, rather than as a separate profession sitting alongside it, since the scope of practice and level of independence differ meaningfully between the two.
Does not apply to UK practice
Someone holding an LMSW is actively working toward full independent licensure, accumulating the supervised hours that will eventually allow them to sit the further examination required for LCSW status. The credential is entirely American and does not apply to practice in the UK, where the social work regulatory structure and career pathway are built quite differently.