The phospholipid l‐α‐lysophosphatidylinositol (
LPI
), an endogenous ligand for
GPR
55, is elevated in patients with acute coronary syndrome, and a
GPR
55 antagonist cannabidiol (
CBD
) reduces experimental ischemia/reperfusion (I/R) injury. While
LPI
activates multiple signaling pathways, little is known about which ones are important in cardiomyocytes. In this study we explored whether activation of the Rho kinase/
ROCK
/p38
MAPK
pathway is responsible for
LPI
‐induced extension of I/R injury. Using a high‐throughput screening method (dynamic mass redistribution;
DMR
), mouse‐ and human‐induced pluripotent stem cell (
iPSC
) cardiomyocytes exposed to
LPI
were shown to exhibit a rapid, sustained, and concentration‐dependent (1 nmol L
−1
‐30 μmol L
−1
) cellular response. Y‐27632 (
ROCK
inhibitor; 10 & 50 μmol L
−1
) and
CBD
(1 μmol L
−1
) both abolished the
DMR
response to
LPI
(10 μmol L
−1
). In murine
iPSC
cardiomyocytes,
LPI
‐induced
ROCK
and p38
MAPK
phosphorylation, both of which were prevented by Y‐27632 and
CBD
, but did not induce
JNK
activation or cleavage of caspase‐3. In hearts isolated from wild type (
WT
) mice subjected to 30 minutes global I/R,
LPI
(10 μmol L
−1
) administered via the coronary circulation increased infarct size when applied prior to ischemia onset, but not when given at the time of reperfusion. The exacerbation of tissue injury by
LPI
was not seen in hearts from
GPR
55
−/−
mice or in the presence of Y‐27632, confirming that injury is mediated via the
GPR
55/
ROCK
/p38
MAPK
pathway. These findings suggest that raised levels of
LPI
in the vicinity of a developing infarct may worsen the outcome of AMI.