SEWM is currently part of the Aspire Group, a provider of housing, training and neighbourhood services, and will become a wholly-owned trading subsidiary of SEUK from the end of next month.
Collectively, they will have a combined membership of about 1,100 social enterprises.
Under the terms of the deal, SEWM will maintain its staff team and offices in Birmingham. SEWM currently employs five staff, according to its website.
Peter Holbrook, chief executive of SEUK, told Third Sector that a fee would be paid to acquire SEWM, but could not say how much it would be because the deal had not yet been completed.
The Aspire Group declined to comment on the terms of the arrangement.
The organisations said in a joint statement that they would consult their members and stakeholders about how to integrate the businesses over the next year.
They said the partnership would enable them to deliver "the support, brokerage, campaigns and information that best serve the needs of their members and the wider social enterprise movement".
Sinéad Butters, chief executive of the Aspire Group, said in a statement: "The SEWM team have done a fantastic job over recent years, delivering real member benefit and leadership to the sector in the region, whilst increasing income year-on-year.
"SEWM and SEUK have worked together successfully on a range of projects in the past five years, including the Buy Social Directory website and Social Saturday. This new partnership with SEUK will only serve to strengthen such work – and the support for social enterprises regionally and nationally."
Holbrook said: "We are very happy to be working so closely with SEWM, which rightly has a fantastic reputation for the work it does in the Midlands and beyond. Together, we believe we can deliver the very best for our social enterprise members. We have much to learn from the pioneering work that the SEWM team has been doing in areas such as social value, housing and membership services."