HESS J1825−137 is a pulsar wind nebula (PWN) whose TeV emission extends across ∼1 $\deg$ . Its large asymmetric shape indicates that its progenitor supernova interacted with a molecular cloud located in the north of the PWN as detected by previous CO Galactic survey (e.g. Lemiere, Terrier & Djannati-Ataï). Here, we provide a detailed picture of the interstellar medium (ISM) towards the region north of HESS J1825−137, with the analysis of the dense molecular gas from our 7 and 12 mm Mopra survey and the more diffuse molecular gas from the Nanten CO(1–0) and GRS 13CO(1–0) surveys. Our focus is the possible association between HESS J1825−137 and the unidentified TeV source to the north, HESS J1826−130. We report several dense molecular regions whose kinematic distance matched the dispersion measured distance of the pulsar. Among them, the dense molecular gas located at (RA, Dec.) = (18h421h,−13 $_{.}^{\circ}$ 282) shows enhanced turbulence and we suggest that the velocity structure in this region may be explained by a cloud–cloud collision scenario. Furthermore, the presence of a H α rim may be the first evidence of the progenitor supernova remnant (SNR) of the pulsar PSR J1826−1334 as the distance between the H α rim and the TeV source matched with the predicted SNR radius R SNR ∼ 120 pc. From our ISM study, we identify a few plausible origins of the HESS J1826−130 emission, including the progenitor SNR of PSR J1826−1334 and the PWN G018.5−0.4 powered by PSR J1826−1256. A deeper TeV study however, is required to fully identify the origin of this mysterious TeV source.