This paper introduces a formal metadata specification for mobility data portals as an extension of DCAT-AP, called mobilityDCAT-AP. It addresses a scenario in which mobility data is offered on a data portal, and is intended to be found, assessed and reused by data users. Unlike in other domains, a structured and community-based metadata for the wider mobility domain has not been established yet. With such specification, an agreed usage of metadata among different portals; easier access to mobility data; improved interoperability in the mobility data eco-system; and the leveraging of semantic technologies are envisioned. In addition, the Resource Description Framework (RDF) as a de-facto standard for metadata, is applied to model the metadata vocabulary. The paper elaborates on the overall goals, previous works on metadata specification and harmonisation, the working process, concrete deliverables and future prospects of mobilityDCAT-AP.