The missive - signed by six Assembly Members (AMs) and Ceredigion MP Mark Williams - called for a reversal of proposed budget cuts to BBC Wales and S4C.
It also said local newspapers should be re-classified as community assets.
The Welsh government said it had "made clear" its own concerns.
The UK government has been asked to comment.
"The media landscape across the UK is dominated by a handful of large corporations who fail to adequately represent the UK population and, in particular, the communities and cultures that exist in Wales," the letter read.
Unions, including the NUJ, Unite, Unison and BECTU, joined The Welsh Language Society as co-signatories, along with a series of media and journalism academics.
AMs from Welsh Labour, Welsh Liberal Democrats and Plaid Cymru also added their names to the letter, issued by the Media Reform Coalition.
An open letter calling on the UK and Welsh governments to prevent "damaging media concentration" has been signed by politicians, unions and academics.