The present invention relates to a rack system and a slide rail assembly. The slide rail assembly includes a first rail, a second rail, a third rail and an extension component; the first rail is provided with a first channel; the second rail is movably displaceable relative to the first rail; the third rail is movably disposed between the first channel of the first rail and the second rail; the extension component is connected with the first rail; and a surface of the extension component and the outer surface of the third rail are substantially coplanar.