Carbon radio recombination lines ( RRLs ) at low frequencies ( \lesssim 500 \textrm { MHz } ) trace the cold , diffuse phase of the interstellar medium , which is otherwise difficult to observe . We present the detection of carbon RRLs in absorption in M82 with LOFAR in the frequency range of 48 - 64 \textrm { MHz } . This is the first extragalactic detection of RRLs from a species other than hydrogen , and below 1 \textrm { GHz } . Since the carbon RRLs are not detected individually , we cross-correlated the observed spectrum with a template spectrum of carbon RRLs to determine a radial velocity of 219 \pm 9 \textrm { km s } ^ { -1 } . Using this radial velocity , we stack 22 carbon- \alpha transitions from quantum levels n = 468 - 508 to achieve an 8.5 \sigma detection . The absorption line profile exhibits a narrow feature with peak optical depth of 3 \times 10 ^ { -3 } and FWHM of 31 \textrm { km s } ^ { -1 } . Closer inspection suggests that the narrow feature is superimposed on a broad , shallow component . The total line profile appears to be correlated with the 21 cm H i line profile reconstructed from H i absorption in the direction of supernova remnants in the nucleus . The narrow width and centroid velocity of the feature suggests that it is associated with the nuclear starburst region . It is therefore likely that the carbon RRLs are associated with cold atomic gas in the direction of the nucleus of M82 .