Self-assembled Ag NPs on DNA chains were synthesized using a simple photochemical technique. The size of the individual Ag NPs and their average diameter on the DNA chains can be tuned just by controlling the various reaction parameters. The average diameter of the self-assembled Ag NPs on DNA chains were varied in the 20–75 nm range, whereas the average diameters of the individual Ag NPs can be varied in the 5–15 nm range. The nominal length of the DNA chains self-assembled by Ag NPs can be extended to a few microns. The present process does not need any external reducing agent like sodium borohydride or hydrazine or others, and DNA itself can act in a dual role: a ‘reducing agent’ and a ‘stabilizing agent’ for the formation of DNA–Ag NPs colloidal dispersion. The synthesized DNA–Ag NPs were well characterized by UV-Vis, EDS, TEM, XRD, XPS, and FT-IR analyses. The self-assembled Ag NPs on DNA chains were found to have good catalytic activity towards the reduction of aromatic nitro compounds. The potentiality of the DNA–Ag NPs as catalyst was tested both in solution and after depositing over solid substrate as thin films. The synthesized Ag NPs on DNA were found to be extremely stable for a period of six months under ambient conditions while kept inside a refrigerator. This pronounced catalytic activity of the material might be useful for future applications in a variety of organic and inorganic catalysis reactions and in SERS-based detection of environmentally important bio-molecules and in sensors.