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Adatoms offer an effective route to modify and engineer the properties of graphene.In this work, we create dilute fluorinated graphene using a clean, controlled and reversible approach. At low carrier densities, the system is strongly localized and exhibits an unexpected, colossal negative magnetoresistance. The zero-field resistance is reduced by a factor of 40 at the highest field of 9 T and shows no sign of saturation. Unusual "staircase" field dependence is observed below 5 K. The magnetoresistance is highly anisotropic. We discuss possible origins, considering quantum interference effects and adatom-induced magnetism in graphene.Defects and adsorbates have proven powerful in altering the electronic properties of graphene through doping, scattering and band gap induction [1][2][3][4][5][6][7][8]. In particular, point defects such as vacancies and adatoms can perturb the electronic states of graphene strongly, leading to midgap states [1] and drastic change of transport properties [4][5][6][7][8].With the ability to control the form and density of defects and the assistance of microscopic imaging tools, defect-engineered graphene can be used as model systems to 2 study the complex role of disorder in two dimensions. Moreover, it has been predicted that point defects may also introduce local magnetic moments and magnetic interactions unavailable in pristine graphene. Calculations show that the peculiar nature of Dirac fermions gives rise to ferromagnetic (antiferromagnetic) interactions among moments occupying the same (opposite) graphene sublattice and possible competing magnetic orders [9,10]. Evidence of magnetism in single-layer graphene remains elusive to date. A controllable defect coverage together with in situ wide tunability of the electronic states makes defective graphene an ideal venue to explore the above novel physical phenomena.In this Letter, we create clean, dilute fluorinated graphene (DFG) and report the observation of unexpected, colossal, negative magnetoresistance (MR). As the electron density is reduced to below the density of fluorine adatoms, the system undergoes a transition from weak to strong localization. In the strongly localization regime, a perpendicular magnetic field drastically reduces the DFG resistance by up to 40-fold at 9 T, while an-plane magnetic field causes only a very small positive MR. We compare our data to existing orbital mechanisms and discuss the possibility of magnetism in our samples.Fluorination controls the properties of carbon materials effectively [11]. Fluorine adatoms are chemically simple and stable in ambient conditions. In this study, we work in the extreme dilute limit such that the properties of graphene dominate. Fluorine adatoms are covalently attached to graphene using CF 4 plasma [12]. This fluorination process is nearly completely reversible. We monitor the concentrations of fluorine adatoms and the unintentionally generated vacancies using Raman spectroscopy (Fig. 1a). With a carefully optimized recipe, the number of vacancies is min...