TITLE: GCN CIRCULAR NUMBER: 21197 SUBJECT: GRB 170604A: GTC spectroscopic redshift DATE: 17/06/05 08:04:01 GMT FROM: Zach Cano at U of Iceland L. Izzo, Z. Cano (IAA-CSIC), A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), D. A. Kann (IAA-CSIC), C. C. Thoene (IAA-CSIC), A. Marante (GTC) and S. Geier (GTC, IAC) report on behalf of a larger collaboration: We observed the afterglow of GRB 170604A (Page et al., GCN Circ. 21191) using OSIRIS at the 10.4 m GTC telescope at La Palma (Spain). The observation consisted of 2x900s and 1x1200s exposures using the R1000B grism, which covers a wavelength range of 3700-7880 AA. Observations began at 04:14:19 UT on the morning of 05 June, 2017, at a mean epoch of +9.51 hrs from the GRB detection. The afterglow candidate (Izzo et al, GCN Circ. 21192) is detected in the r-band acquisition image, where we measure r = 20.3 +- 0.1 relative to nearby PANSTARRS standards in the GRB field of view. The afterglow spectrum shows a continuum through the observed wavelength range, and we identify absorption features corresponding to MgII, MgI, and FeII at a common redshift of z=1.329, which we identify as the redshift of GRB 170604A.