Background/aim: The purpose of this study was to determine the effects of the supplemental feeding tube device (SFTD) and bottle methods on weight gain, transition to full breastfeeding, breastfeeding success, and duration of discharge in preterm infants. Materials and methods: This randomized controlled trial was conducted with a total of 46 preterm infants including 23 infants in study (SFTD) and control (bottle) groups. An information form, an infant follow-up form for feeding, and LATCH breastfeeding assessment instrument were used to collect the data. Results: The gestation week of the infants in the study group was 31.22 ± 2.76, and in the control group it was 30.52 ± 2.47. The birth weight of the infants in the study group was 1586.3 ± 525.35 g and 1506.09 ± 454.77 g in the control group. The daily weight gain of the infants was 24.09 ± 15.21 g in the study group and 27.17 ± 17.63 g in the control group. The infants in the study group (4.70 ± 2.44 days) transitioned to full breastfeeding earlier than those in the control group (6.00 ± 4.10 days). LATCH 2nd measurement scores were significantly higher in both groups than LATCH 1st measurement scores (p < 0.01). Although it was not statistically significant (p > 0.05), the infants in the study group (10.22 ± 5.20 days) were discharged earlier than those in the control group (13.48 ± 8.78 days). Conclusion: The SFTD and bottle methods were determined to be similar in terms of daily weight gain, transition to full breastfeeding, breastfeeding success, and duration of hospitalization.Key words: Bottle feeding, breastfeeding, supplemental feeding methods, neonatal intensive care unit, premature