PRS Media, trading as Purus Digital and based in Dale, was fined £140,000 by the Information Commissioner's Office (ICO).
The information it used to send texts was taken from its own competition website where people had to agree to marketing as a condition of entering.
The action was prompted by complaints from members of the public.
The ICO said the condition on its website did not amount to consent as the law states people must have a genuine choice over whether or not to consent to marketing.
Ken Macdonald, head of ICO regions, said: "Nuisance marketing, whether it's by calls to people's landline or mobile, or through spam texts, causes disruption, annoyance and, in the worst cases, serious upset.
"We issue fines like these to firms behind nuisance marketing to send a clear message that such action will not be tolerated."
The company could not be contacted for a comment.
A Pembrokeshire company which flouted data regulations by sending over four million spam texts has been fined.