  Using LOFAR, we have performed a very-low-frequency (115-155 MHz) radio
survey for millisecond pulsars (MSPs). The survey targeted 52 unidentified
Fermi $\gamma$-ray sources. Employing a combination of coherent and incoherent
dedispersion, we have mitigated the dispersive effects of the interstellar
medium while maintaining sensitivity to fast-spinning pulsars. Toward 3FGL
J1553.1+5437 we have found PSR J1552+5437, the first MSP to be discovered
(through its pulsations) at a radio frequency $<$ 200 MHz. PSR J1552+5437 is an
isolated MSP with a 2.43 ms spin period and a dispersion measure of 22.9 pc
cm$^{-3}$. The pulsar has a very steep radio spectral index ($\alpha < -$2.8
$\pm$ 0.4). We obtain a phase-connected timing solution combining the 0.74
years of radio observations with $\gamma$-ray photon arrival times covering 7.5
years of Fermi observations. We find that the radio and $\gamma$-ray pulse
profiles of PSR J1552+5437 appear to be nearly aligned. The very steep spectrum
of PSR J1552+5437, along with other recent discoveries, hints at a population
of radio MSPs that have been missed in surveys using higher observing
frequencies. Detecting such steep spectrum sources is important for mapping the
population of MSPs down to the shortest spin periods, understanding their
emission in comparison to slow pulsars, and quantifying the prospects for
future surveys with low-frequency radio telescopes like SKA-Low and its
precursors.
