Here we report our discovery of a band of blue luminescence (BL) in the Red Rectangle nebula. This enigmatic proto-planetary nebula is also one of the brightest known sources of extended red emission as well as of unidentified infrared (UIR) band emissions. The spectrum of this newly discovered BL is most likely fluorescence from small neutral polycyclic aromatic hydrocarbon (PAH) molecules. PAH molecules are thought to be widely present in many interstellar and circumstellar environments in our Galaxy as well as in other galaxies and are considered likely carriers of the UIR band emission. However, no specific PAH molecule has yet been identified in a source outside the solar system because the set of mid-infrared emission features attributed to these molecules between the wavelengths of 3.3 and 16.4 μm is largely insensitive to molecular sizes. In contrast, near-UV/blue fluorescence of PAHs is more specific as to size, structure, and charge state of a PAH molecule. If the carriers of this near-UV/blue fluorescence are PAHs, they are most likely neutral PAH molecules consisting of three to four aromatic rings such as anthracene (C₁₄H₁₀) and pyrene (C₁₆H₁₀). These small PAHs would then be the largest molecules specifically identified in the interstellar medium.