We report about the anomalous Hall effect in 4f 3d metallic alloys measured using terahertz time-domain spectroscopy. The strength of the observed terahertz spin-dependent transport phenomenon is in good agreement with expectations based on electronic transport measurements. Employing this effect, we succeeded to reveal ultrafast dynamics of the anomalous Hall effect which accompanies the sub-100 picosecond optically induced magnetization reversal in a GdFeCo alloy. The experiments demonstrate the ability to control currents at terahertz frequencies in spintronic devices magnetically and ultrafast.To develop ultrafast spintronics logic protocols for high-speed data processing, one has to find a way to control and detect the spin-dependent transport ultrafast. The conventional approach to study magnetotransport is based on the application of an electrical voltage to a sample and the detection of the subsequent electrical current via electrodes. However, making electrical contacts that transfer a picosecond (ps) short signal without substantial distortions and broadening is rather challenging [1]. An elegant solution for the problem of ultrafast conductivity measurements can be found by using freely propagating terahertz (THz) waves.To date, THz spectroscopy has been used to study magnetotransport in a large diversity of materials [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. However, only a few of these studies have been dedicated to magnetically ordered materials and in none of these materials the magnetization could be reversed ultrafast. To reveal sub-100 ps dynamics in the anomalous Hall effect, we have chosen ferrimagnetic Gdx(FeCo)1-x thin films, as they are expected to show a significant anomalous Hall effect [18][19][20], and are known for their ultrafast all-optical magnetization reversal [21][22][23][24][25][26].The studied Gdx(Fe0.875Co0.125)1-x samples are 20 nm thick amorphous films, which have adjacent 5 nm SiN protecting layers and are deposited using magnetron sputtering. These samples have a ferrimagnetic ordering and an out-of-plane magnetic anisotropy in the studied Gd concentration range (0.2