CH3NH3PbBr3 perovskite quantum dots (PQDs) are synthesized by using four different linear alkyl phosphonic acids (PAs) in conjunction with (3‐aminopropyl)triethoxysilane (APTES) as capping ligands. The resultant PQDs are characterized by means of XRD, TEM, Raman spectroscopy, FTIR spectroscopy, UV/Vis, photoluminescence (PL), time‐resolved PL, and X‐ray photoelectron spectroscopy (XPS). PA chain length is shown to control the PQD size (ca. 2.9–4.2 nm) and excitonic absorption band positions (λ=488–525 nm), with shorter chain lengths corresponding to smaller sizes and bluer absorptions. All samples show a high PL quantum yield (ca. 46–83 %) and high PL stability; this is indicative of a low density of band gap trap states and effective surface passivation. Stability is higher for smaller PQDs; this is attributed to better passivation due to better solubility and less steric hindrance of the shorter PA ligands. Based on the FTIR, Raman, and XPS results, it is proposed that Pb2+ and CH3NH3+ surface defects are passivated by R−PO32− or R−PO2(OH)−, whereas Br− surface defects are passivated by R−NH3+ moieties. This study establishes the combination of PA and APTES ligands as a highly effective dual passivation system for the synergistic passivation of multiple surface defects of PQDs through primarily ionic bonding. PDQ synthesis of PQDs: The passivation strategy reported herein is to use organic molecules linear alkyl phosphonic acids (PAs) and (3‐aminopropyl)triethoxysilane (APTES) as ligands. The PAs and APTES produce R−PO2(OH)−, R−PO32−, and R−NH3+ to effectively passivate the charged surface defects of CH3NH3PbBr3 perovskite quantum dots (PQDs) due to dangling bonds related to species such as MA+, Pb2+ and X−, respectively (see figure).