Fingolimod (FNGL) is an immune-modulatory agent prescribed for relapsing forms of multiple sclerosis. Because of its mechanism of action, FNGL is potentially a treatment for chronic, non-curable T-lymphocyte-driven inflammatory skin diseases (TLDISD) such as psoriasis and atopic dermatitis. Since severe side effects limit the systemic administration of FNGL, the objective of this study is to develop a hydroxypropyl cellulose (2%) FNGL gel for dermatological applications. First, the effect of FNGL strength (0.05%, 0.10%, 0.50%, and 1.00%) on skin permeability and retention was investigated. We carried out several permeation studies with vertical Franz diffusion cells and (i) cellulose or (ii) excised dorsal porcine ear skin (EDPES) as membrane. We also quantified FNGL in the stratum corneum and in dermis with the tape-stripping method. Permeability parameters as well as the amount retained in skin increased significantly (p < 0.01) with strength; however, there was no statistically significant difference between the 0.50% and 1.00% gels for both cellulose and EDPES. Therefore, we selected the 0.50% gel to investigate the effect of colloidal oatmeal (0%, 1%, 3%, 6%, and 10%) on FNGL in vitro permeability and skin retention. Colloidal oatmeal has beneficial dermatological properties for TLDISD and may complement FNGL activity. Permeability increased significantly (p < 0.001) with colloidal oatmeal at the 6% and 10% strength with an enhancement ratio of 3.5 and 2.4, respectively, whereas the amount retained in the skin decreased significantly (p < 0.001) compared to the base gel. In conclusion, the 0.50% FNGL(.)HCL gel with 6% Aveeno® has very promising permeability characteristics for delivery of FNGL to the skin.