This paper investigated the effect of kangaroo mother care (KMC) in the early postpartum period on cortisol levels and immune factors in breast milk. This quasi-experimental study was conducted at the obstetrics clinic of a university hospital in western Türkiye. The sample consisted of 63 mothers and their infants. All mothers had a cesarean delivery. Participants were divided into control (n = 32) and experimental groups (n = 31). The control group received routine care at the clinic. The experimental group received KMC for the first 3 days after birth in addition to the routine care at the clinic. Milk samples were collected on the third day after delivery to examine cortisol, IgA, IgM, and IgG levels. All parameters were measured using the enzyme-linked immunosorbent assay method. The experimental group had lower cortisol levels (17.740 ± 1.438) than the control group (18.503 ± 1.449) (p <.05). This result showed that the difference between the two groups was clinically significant (effect size =.53). There was no significant difference in IgA, IgM, and IgG levels between the groups (p >.05). The experimental and control groups had similar immunological factors, but the former had lower cortisol levels than the latter. Therefore, healthcare professionals should encourage mothers to provide KMC to their infants as soon as possible.