The mailings, which contain a message written from the perspective of Snowball, a Samoyed, asks the question, "Will I ever find that someone special?" and features the strapline, "This Valentine's Day, you can help turn lonely hearts into happy hearts."
The campaign aims to address the wellbeing of the UK's 14,000 stray and abandoned dogs that the charity helped to save last year, and asks recipients to make a donation so it can continue its work.
Dogs Trust said the mailings would profile a number of dogs in the style of a lonely hearts column, including Tizzy, a Jack Russell who needs a donation of £12 because his elderly owner died.
Phil White, business director at 141 Worldwide, said: "Our special understanding of Dogs Trust supporters means that we know how to communicate to them in an appropriate and effective manner so that they can rest assured that Dogs Trust ensures that each and every dog in its care gets all the love and attention it deserves.
"Valentine's Day is the perfect time to communicate to donors the compassion that Dogs Trust provides throughout its network of 17 re-homing centres."
The charity, which was formerly known as the National Canine Defence League, was founded in 1891 and provides kennels for dogs which have been made homeless or abandoned.