One source said the decision to end the relationship was "mutual" and "amicable".
In January it was announced that FleishmanHillard would be handling work across the UK, Germany, Ireland and Poland for the store plus corporate comms for parent company TJX Europe.
John Saunders, FleishmanHillard regional president for EMEA, said: "I can confirm that our assignment for TK Maxx will be coming to an end - but not until the middle of next year so there is a lot of work to be done between now and then.
"It is a great company, a fabulous brand with brilliant people and we have learned a great deal from what has been an exhilarating experience for our people from London to Ireland as well as in Germany and Poland."
TJX Europe was unavailable for comment on the reasons for the relationship ending and whether the brief would be put out to tender.
The brief covered consumer brand PR and press office, social media and content marketing as the retailer seeks to develop comms beyond its core messaging on discount prices.
It is understood that FleishmanHillard was handed the work following a four-way pitch.