We report the serendipitous detection of a significant overdensity of Herschel-Spectral and Photometric Imaging Receiver 250 μm sources in the vicinity of MRC 1138-26. We use an adaptive kernel density estimate to quantify the significance, including a comparison with other fields. The overdensity has a size of ˜3.5-4 arcmin and stands out at ˜5σ with respect to the background estimate. No features with similar significance were found in four Herschel-observed extragalactic control fields: GOODS-North, Lockman, COSMOS and UDS. The chance of having a similar overdensity in a field with the same number but randomly distributed sources is less than 2 per cent. The clump is also visible as a low-surface-brightness feature in the Planck 857 GHz map. We detect 76 sources at 250 μm (with a signal-to-noise ratio greater than 3), in a region of 4 arcmin radius (or 1.51 ± 0.17 arcmin-2); 43 of those (or 0.86 ± 0.13 arcmin-2) are above a flux density limit of 20 mJy. This is a factor of 3.6 in excess over the average 0.24 ± 0.02 arcmin-2 in the four control fields, considering only the sources above 20 mJy. We also find an excess in the number counts of sources with 250 μm flux densities between 30 and 40 mJy, compared to deep extragalactic blank-field number counts. Assuming a fixed dust temperature (30 K) and emissivity (β = 1.5), a crude, blackbody-derived redshift distribution, zBB, of the detected sources is significantly different from the distributions in the control fields and exhibits a significant peak at zBB ≈ 1.5, although the actual peak redshift is highly degenerate with the temperature. We tentatively suggest, based on zBB and the similar S250/S350 colours of the sources within the peak, that a significant fraction of the sources in the clump may be at a similar redshift. Since the overdensity lies ˜ 7 arcmin south of the z = 2.16 Spiderweb protocluster MRC 1138-26, an intriguing possibility (that is presently unverifiable given the data in hand) is that it lies within the same large-scale structure.