We report on the first X-ray observation of the 0.28 s isolated radio pulsar PSR J1154--6250 obtained with the XMM-Newton observatory in February 2018. A point-like source is firmly detected at a position consistent with that of PSR J1154--6250. The two closest stars are outside the 3$sigma$ confidence limits of the source position and thus unlikely to be responsible for the observed X-ray emission. The energy spectrum of the source can be fitted equally well either with an absorbed power-law with a steep photon index $Gammaapprox 3.3$ or with an absorbed blackbody with temperature $kT=0.21pm 0.04$~keV and emitting radius $R_mathrm{BB} approx 80$ m (assuming a distance of 1.36~kpc). The X-ray luminosity of $4.4times 10^{30}$ erg s$^{-1}$ derived with the power-law fit corresponds to an efficiency of $eta_X = L^mathrm{unabs}_X/dot E = 4.5times 10^{-3}$, similar to those of other old pulsars. The X-ray properties of PSR J1154--6250 are consistent with an old age and suggest that the spatial coincidence of this pulsar with the OB association Cru OB1 is due to a chance alignment.
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