He also said that if India had joined the Regional Comprehensive Economic Partnership (RCEP), farmers, labourers and industries would have been adversely affected.
"At the recent ASEAN Summit in Bangkok, our prime minister took a very important decision that India will not be part of RCEP. This is not a small thing," Singh said after inaugurating the Sisseri Bridge here.
The 200-metre long permanent bridge connects the Siang Valley and the Dibang Valley in Arunachal Pradesh.
"Keeping in mind India's economic interests, especially the economic interests of the northeast, the prime minister took this decision. Everyone should welcome this decision by the prime minister," Singh said.
After years of negotiations, India had pulled out of the China-backed mega RCEP over unresolved "core concerns", with Prime Minister Modi saying in Bangkok that the proposed deal would have adverse impact on the lives and livelihoods of all Indians.
Singh said,"He (the prime minister) said we will participate in RCEP at the right moment and it won't be as a small partner but only as a big partner. We will be partners of the same level."
"Farmers, labourers, factories and industries would have got affected had India joined the RCEP. This is a big decision. In this decision, the northeast's interests were kept in mind," he said.
The defence minister said the government has started several infrastructure projects in Arunachal Pradesh and no part of the state will be isolated like "Kala Pani".