The findings, published in the journal Psychoneuroendocrinology, suggest that women who were satisfied in their relationships also reported lower psychological stress - and these two factors were associated with lower markers for inflammation in their blood.
Keeping inflammation at bay is the key to promoting health generally, and especially in breast cancer survivors, researchers said.
"Our findings suggest that this close partnership can boost their bond as a couple and also promote survivors' health even during a very stressful time when they're dealing with cancer," said study lead author Rosie Shroutv from the Ohio State University in the US.
For the findings, the 139 women with an average age of 55 completed self-report questionnaires and provided blood samples at three visits: upon recruitment within one to three months of their cancer diagnosis and during two follow-up visits six and 18 months after their cancer treatment ended.
One survey assessed relationship satisfaction by asking the women to report their degree of happiness, the level of warmth and comfort they felt with their partner, how rewarding the relationship was and their overall satisfaction.
The other questionnaire was used to evaluate their level of perceived psychological stress over the previous week.
Researchers analysed blood samples for levels of four proteins that promote inflammation throughout the body even when there is no need for an immune response.
This kind of chronic inflammation is linked to numerous health problems, including heart disease, Type 2 diabetes, arthritis and Alzheimer's disease, as well as the frailty and functional decline that can accompany ageing.
The findings showed a clear trend in the women as a group: The more satisfied they felt about their romantic relationships, the lower their perceived stress and the lower their inflammation.
The design of the study allowed researchers to compare the group of women to each other and also gauge changes in each woman individually.
"This gave us a unique perspective - we found that when a woman was particularly satisfied with her relationship, she had lower stress and lower inflammation than usual - lower than her own average," Shrout said.
"At a specific visit, if she was satisfied with her partner, her own inflammation was lower at that visit than at a different visit when she was less satisfied," she added.
The research shows the importance of fostering survivors' relationships. It could promote their health over the long run," the authors wrote.