  Large scale two-dimensional (2D) moir\'e superlattices are driving a
revolution in designer quantum materials. The electronic interactions in these
superlattices, strongly dependent on the periodicity and symmetry of the
moir\'e pattern, critically determine the emergent properties and phase
diagrams. To date, the relative twist angle between two layers has been the
primary tuning parameter for a given choice of constituent crystals. Here, we
establish strain as a powerful mechanism to in-situ modify the moir\'e
periodicity and symmetry. We develop an analytically exact mathematical
description for the moir\'e lattice under arbitrary in-plane heterostrain
acting on any bilayer structure. We demonstrate the ability to fine-tune the
moir\'e lattice near critical points, such as the magic angle in bilayer
graphene, or fully reconfigure the moir\'e lattice symmetry beyond that imposed
by the unstrained constituent crystals. Due to this unprecedented simultaneous
control over the strength of electronic interactions and lattice symmetry, 2D
heterostrain provides a powerful platform to engineer, tune, and probe strongly
correlated moir\'e materials.
