The Taunton-based weekly was first published on December 31, 1836, and has produced a supplement to mark the milestone in its history.
The PM wrote: “I am delighted to send my congratulations and best wishes to all the staff and readers of the Somerset County Gazette on the paper’s 170th anniversary.
“There can be few papers in the country which can match such a record of service to its community.
“We are very fortunate in this country in having many superb local papers. You play a vital role in informing your readers, and reflecting their concerns and hopes, and, in doing so, helping to bind our communities and our country together.
“Congratulations again on reaching this landmark and best wishes for the next 170 years.”
Messages have also been received from local MPs, civic dignitarites and celebrities including Taunton-born author Arthur C. Clarke, who now lives in Sri Lanka, all of which were published in the supplement.
Editor-in-chief Ken Bird, who joined the paper as a trainee reporter in 1980, said: “During its long and illustrious history, the County Gazette has won a string of awards as well as raising many hundreds of thousands of pounds for worthwhile causes.
“Of course, none of this could be achieved without the efforts of hard-working staff and the support of both readers and advertisers.
“We are proud to be the paper of record for our area.”