The court has held that the prosecution had failed to prove the charges against the accused.
The judgment was delivered by principal district and sessions judge N Rathinaraj.
Three persons, including a security guard and two employees of the daily, were killed when alleged DMK activists attacked the office and set it on fire on May 9, 2007, angered by an opinion poll in the Dinakaran which gave Chief Minister M Karunanidhi's younger son M K Stalin higher popularity rating than his brother M K Azhagiri.
Sixteen persons, suspected to be supporters of Azhagiri, an Union minister, had been charged with various offences including murder and rioting while deputy superintendent of police V Rajaram was named as an accused for alleged dereliction of duty.
The case, initially handled by the local police, was transferred to the CBI on the orders of Karunanidhi later. The CBI included the DSP as an accused.
The attack on the daily owned by Kalanidhi Maran, brother of Union Textiles Minister Dayanidhi Maran, had led to the latter's resignation from the Union Cabinet after they fell out with the Karunanidhi family.
However, the differences were sorted out before the Lok Sabha polls this year, which facilitated the return of Dayanidhi Maran to the Lok Sabha and the Union Cabinet.