Given the high fluctuations of the general price level, the importance of investment, and accessing sufficient financial resources, we examine the effects of monetary policy on the nominal (Inflation) and real (Investment) sectors of a developing economy, like Iran, through the channel of the relationship between bank deposits, stock market, and speculation in the foreign exchange market (FEM). For this purpose, due to regime switching in Iran’s economy, the non-linear relation between the variables has been investigated by applying Markov switching models and annual data of 1988–2018. The results show that the financial intermediaries (FIs) have not had a complementary relationship in both bear and bull market and high and low banks deposits regimes. Both FIs have negatively impacted speculation in the FEM in high and low exchange rates difference regimes. The difference between official and unofficial exchange rates negatively impacted the FIs in bear market and low banks deposits regimes; on the contrary, a positive effect in a bull market and high bank deposits. The effects of both FIs on investment have been positive in high and low investment regimes and negative on inflation rates in high and very high inflation regimes. In comparison, the impact of speculation in the FEM has been negative on investment and positive on inflation. Therefore, in the event of the lack of limitations regarding using interest rate (IR) and the policy of exchange rate unification, the central bank would be able to affect the nominal and real sectors of the economy suitably through the channels of the FIs and the FEM.