Kashyap beat his 14th seeded Thai opponent 21-19, 14-21, 21-17 in a contest which lasted one hour and 12 minutes at the packed BBD UP Academy courts.
Compatriot PV Sindhu, however, had to be content with the runners-up finish in the women's singles.
Kashyap, the London Olympics quarterfinalists and seeded second in the tournament clinched the first game, lost a bit of concentration in the second, but ensured in the decider that the national anthem was played to the joy of the Sunday crowd.
"I am very thrilled to win my first Indian GP title. I give all the credit to Gopi Sir (India's chief coach Pulella Gopichand)," said Kashyap who celebrated his victory by punching the air several times before throwing his racquet into the crowd.
Kashyap, the highest ranked Indian man shuttler in the BWF rankings, and Saensomboonsuk went neck to neck in all the three games, which is also reflected in the final score.
The Indian used his lethal smashes to maximum effect, had better court coverage, was deceptive in his shot selection which helped him come from behind, from 7-11 in the first game to lead 18-17.
On the other hand, Saensomboonsuk's gameplan was hinged on strong net play and he controlled the second game well to restore parity before Kashyap settled the issue in the decider.
Earlier in the day, seventh seeded Indonesian Lindaweni Fanetri upset Sindhu, the second seed, in the women's final 21-15, 18-21, 21-18.
Coach Gopichand said, "I have a mixed feeling - Kashyap won but Sindhu lost her final. I'm overwhelmed with Kashyap's victory. Sindhu showed good fitness and am glad that she reached the final here."
Himachal Pradesh, Assam and Delhi scored victories over their respective opponents in the group stage matches of the 63rd Senior National Basketball ...