We have prepared a large volume of pure, concentrated and homogeneous magnesium standard solution (GSB-Mg) to be used as a secondary reference standard by the magnesium isotope community. This new standard solution can also be used for quality assurance, including the development and validation of analytical procedures, preparation and test of analytical methods, quality control and training of analysts. The delta Mg-26 values relative to DSM3, measured by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), are -2.038 +/- 0.027 parts per thousand (n= 45, Lab 1), -2.024 +/- 0.055 parts per thousand (n= 7, Lab 2), -2.078 +/- 0.081 parts per thousand (n= 6, Lab 3), -2.037 +/- 0.015 parts per thousand (n= 8, Lab 4), and -2.067 +/- 0.039 parts per thousand (n= 12, Lab 5). The delta Mg-26 and delta Mg-25 values for the GSB-Mg standard solution are -2.049 parts per thousand and -1.056 parts per thousand, with a combined expanded (k= 2) uncertainty of 0.049 parts per thousand and 0.028 parts per thousand, respectively. These values show higher similarity to Mg isotopic compositions of carbonates and marine sediments compared to those of the mantle and crustal rocks, making the GSB-Mg solution a great reference material for Mg isotopic analysis of carbonates and marine sediments.