In situ-forming hydrogels that possess the ability to be injected in a less invasive manner and mimic the biochemical composition and microarchitecture of the native cartilage extracellular matrix are desired for cartilage tissue engineering. Besides, gelation time and stiffness of the hydrogel are two interdependent factors that affect cells' distribution and fate and hence need to be optimized. This study presented a bioinspired in situ-forming hydrogel composite of hyaluronic acid (HA), chondroitin sulfate (CS), and collagen short nanofiber (CSNF). HA and CS were functionalized with aldehyde and amine groups to form a gel through a Schiff-base reaction. CSNF was fabricated via electrospinning, followed by fragmentation by ultrasonics. Gelation time (11–360 s) and compressive modulus (1.4–16.2 kPa) were obtained by varying the concentrations of CS, HA, CSNFs, and CSNFs length. The biodegradability and biocompatibility of the hydrogels with varying gelation and stiffness were also assessed in vitro and in vivo. At three weeks, the assessment of hydrogels' chondrogenic differentiation also yields varying levels of chondrogenic differentiation. The subcutaneous implantation of the hydrogels in a mouse model indicated no severe inflammation. Results demonstrated that the injectable CS/HA@CSNF hydrogel was a promising hydrogel for tissue engineering and cartilage regeneration.