Dopamine system disorders ranging from movement disorders to addiction and schizophrenia involve striatal medium spiny neurons (MSNs), yet their functional connectivity has been difficult to determine comprehensively. We generated a mouse with conditional channelrhodopsin-2 expression restricted to medium spiny neurons and assessed the specificity and strength of their intrinsic connections in the striatum and their projections to the globus pallidus and the substantia nigra. In the striatum, medium spiny neurons connected with other MSNs and tonically active cholinergic interneurons, but not with fast-spiking GABA interneurons. In the globus pallidus, medium spiny neurons connected strongly with one class of electrophysiologically identified neurons, but weakly with the other. In the substantia nigra, medium spiny neurons connected strongly with GABA, but not with dopamine neurons. Projections to the globus pallidus showed solely D2-mediated presynaptic inhibition, whereas projections to the substantia nigra showed solely D1-mediated presynaptic facilitation. This optogenetic approach defines the functional connectome of the striatal medium spiny neuron.