Retinogeniculate (RG) synapses connect retinal ganglion cells to the thalamic relay cells of the dorsal lateral geniculate nucleus (dLGN). They are critical for regulating the flow of visual information from retina to primary visual cortex (V1). RG synapses in dLGN are uniquely larger and stronger than their counterparts in other retinorecipient regions. Moreover, in dLGN, RG synapses can be classified into two groups: simple RG synapses, which contain glia-encapsulated single RTs synapsing onto relay cell dendrites, and complex RG synapses, which contain numerous RTs that converge onto the shared regions of relay cell dendrites. To identify target-derived molecules that direct the transformation of RTs into unique RG synapses in dLGN, I used RNAseq to obtain the whole transcriptome of dLGN and its adjacent retinorecipient nucleus, vLGN, at different time points during RG synapses development. Leucine-Rich Repeat Transmembrane Neuronal 1 (LRRTM1), a synaptogenic adhesion molecule, was the candidate I selected based on its expression pattern. Here, I discovered that LRRTM1 regulates the development of complex RG synapses. Mice lacking LRRTM1 (lrrtm1-/-) not only show a significant reduction in the number of complex RG synapses but they exhibit abnormal visual behaviors. This work reveals, for the first time, a high level of retinal convergence onto dLGN relay cells in thalamus and the functional significance of this convergence for vision.