This paper analyzes the welfare implications of a linear financial transactions tax in a multi-period portfolio selection model with heterogeneous agents. Under certain conditions over the redistribution of government revenues, with no Pigouvian motives, I find such a tax may induce first-order losses due to the distortion of idiosyncratic risk-sharing. A transaction tax induces both a contemporaneous inaction region and intertemporal shifting of transaction realizations. Given a segmented market over multiple trading platforms and the choice of at least two different tax instruments, I develop sufficient statistic formulas that differ from standard optimal commodity tax theory in that they account for untaxed capital accumulation, market structure rents, and uncertain returns.