The association of vitamin D with neuro-behavioral outcomes of young children is unclear, particularly of those who reside in tropical countries and are otherwise exposed to adequate sun light. To investigate this association, we analysed the existing data of poor infants, who participated in an observational, prospective-cohort (MalED) study, conducted in a slum of Dhaka-city. We assessed 265 infants aged 6-8 months for cognitive, motor, language and behavior development using Bayley-III. Information about infants' temperament and communicative skills were provided by the mothers through a culturally modified "temperament-scale" and a "communicative-developmental inventory". Serum concentration of vitamin D [25(OH)D] was measured in 205 infants. Around 28.3% of infants in this community had low level vitamin D, with the cut-off at <50 nmol/L. After controlling for all possible covariates, a multivariable-adjusted linear regression showed that children with vitamin D levels <50 nmol/L had significantly lower scores in two dimensions of temperament: activity (B±SE 1.64±0.78; 95%CI 0.10, 3.18; p = 0.037; effect size 0.37 SDs) and soothabilty (2.02±0.70; 0.64, 3.41; p = 0.004; 0.53 SDs), compared to children with vitamin D levels of ≥50nmol/L. These infants also scored low in word comprehensions (1.28±0.62; 0.05, 2.51; p = 0.042; 0.23 SDs) and were less active during test-procedures (0.33±0.16; 0.02, 0.64; p = 0.035; 0.27 SDs). Both the groups tested similarly in cognitive and motor scores. This study found, despite adequate sunlight-exposure, one in four infants of this slum-community are suffering from a subclinical vitamin D deficiency. Higher levels of vitamin D in these infants showed a positive association with temperament, language and behavior but not with cognitive and motor development. Our findings highlight the early-detected extra-skeletal neuro-behavioral role of vitamin D. Future studies in this area will give more insight.