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Epub 2021 Jun 12. Significance: Early monolingual versus bilingual experience induces adaptations in the development of linguistic and cognitive processes, and it modulates functional activation patterns during the first months of life. Resting-state functional connectivity (RSFC) is a convenient approach to study the functional organization of the infant brain. RSFC can be measured in infants during natural sleep, and it allows to simultaneously investigate various functional systems. Adaptations have been observed in RSFC due to a lifelong bilingual experience. Investigating whether bilingualism- induced adaptations in RSFC begin to emerge early in development has important implications for our understanding of how the infant brain’s organization can be shaped by early environmental factors. Aims: We attempt to describe RSFC using functional near-infrared spectroscopy (fNIRS) and to examine whether it adapts to early monolingual versus bilingual environments. We also present an fNIRS data preprocessing and analysis pipeline that can be used to reliably characterize RSFC in development and to reduce false positives and flawed results interpretations. Methods: We measured spontaneous hemodynamic brain activity in a large cohort (N ¼ 99) of 4-month-old monolingual and bilingual infants using fNIRS. We implemented group-level approaches based on independent component analysis to examine RSFC, while providing proper control for physiological confounds and multiple comparisons. Results: At the group level, we describe the functional organization of the 4-month-old infant brain in large-scale cortical networks. Unbiased group-level comparisons revealed no differences in RSFC between monolingual and bilingual infants at this age. Conclusions: High-quality fNIRS data provide a means to reliably describe RSFC patterns in the infant brain. The proposed group-level RSFC analyses allow to assess differences in RSFC across experimental conditions. An effect of early bilingual experience in RSFC was not observed, suggesting that adaptations might only emerge during explicit linguistic tasks, or at a later point in development. The authors would like to thank all the parents and infants who generously participate in our studies. The authors also would like to thank Elena Aguirrebengoa for her assistance on recruiting and testing participants and Enrico Amico for discussion regarding the connICA approach. This work was supported by the Basque Government (Nos. PRE_2018_2_0154, PIBA_2019_ 104, and BERC 2018-2021); the Spanish Ministry of Economy and Competitiveness (Nos. RYC-2017-21845, PID2019-105520GB-100, and PSI2014-5452-P); the Natural Sciences and Engineering Research Council of Canada (Nos. 506948 and 506993); and the Engineering and Physical Sciences Research Council (Nos. EP/N025946/1 and EP/509577/1).