  We report the discovery of the classical Cepheid OGLE-GD-CEP-1884 (=
GDS_J1535467-555656) with the longest pulsation period known in our Galaxy. The
period of 78.14 d is nearly 10 d longer than that of the previous
record-holding Cepheid, S Vulpeculae, and thus, OGLE-GD-CEP-1884 can be
categorized as the first ultra long period Cepheid in the Milky Way. This star
is present in the ASAS-SN and Gaia DR3 catalogs of variable stars, but it has
been classified as a long-period variable in those catalogs. Based on more than
10 years of the photometric monitoring of this star carried out by the OGLE
project in the I and V bands and a radial velocity curve from the Gaia Focused
Product Release, we unequivocally demonstrate that this object is a
fundamental-mode classical Cepheid. By employing the mid-infrared
period-luminosity relation, we determine the distance to OGLE-GD-CEP-1884 (4.47
+- 0.34 kpc) and place it on the Milky Way map, along with about 2400 other
classical Cepheids. We also discuss the potential of finding additional ultra
long period Cepheids in our Galaxy.
