The multi-year deal includes measurement for Sinclair’s local stations, digital broadcast networks and national unwired network.
Financial terms were not disclosed.
"We're pleased to have reached a mutually beneficial agreement with Nielsen," said Steve Marks, executive VP & COO of Sinclair. "We are happy to continue our long-standing relationship, and look forward to Nielsen's investments in Local broadcasting, as well as its future support of ATSC 3.0."
Sinclair is one of the largest station owners and is in the middle of acquiring Tribune’s stations as well.
Local measurement has been a strength for comScore as it tries to challenge Nielsen in the measurement market.
"Sinclair's content plays a critical role for the communities in which it serves," said Megan Clarken, president of Nielsen Watch. "We're delighted to continue our relationship with Sinclair and to support the countless growth opportunities involved in the deployment of ATSC 3.0."