Fast response of a photodetector is requisite for many advanced applications such as optical communication, biological sensing, missile tracking, and so on. In this work, we further investigated response speed of the Gr/MoSe2/Si heterojunction photodetector by using an oscilloscope to monitor variation of photovoltage under pulsed red light illumination (Figure 5 a). The pulsed red light (650 nm) was produced from a laser diode (LD) supplied by a tuneable square‐wave signal generator at 14 V. Figure 5b,c and Figure S4 in the Supporting Information show the photoresponse of the Gr/MoSe2/Si photodetector to pulsed light with frequency ranging from 1 kHz to 1 MHz, manifesting that the device can work well even for high frequency (MHz) optical signals. In addition, from the enlarged photoresponse curve at 1 MHz, Figure 5d, rise time (t r) and fall time (t f) are estimated to be 270 and 350 ns, respectively. The response time of the LD light source is ≈6 ns (Figure 5d), ensuring the accurate evaluation of the device response time. It is noteworthy that this response speed is much quicker than the current reports on MoSe2‐based photodetectors, and even faster than other MoS2‐ or graphene‐based photodetectors (Table 1). This ultrafast response speed can be attributed to the unique device structure of the Gr/MoSe2/Si photodetector: (i) Unlike the conventional photoconductors or phototransistors with MSM structures, whose response speed is usually limited by the transmit time of the carriers between two contacts and the defects/traps in the conduction channels, strong built‐in electric field in the Gr/MoSe2/Si heterojunction photodetector can greatly facilitate the separation and transport of photogenerated carriers. (ii) The distinct vertically standing layered structure of MoSe2 film ensures the fast transport of photogenerated carriers along the vertical direction due to the high in‐plane mobility. The transmit time should be less than 1 ns from the junction interface to the top electrode by assuming a mobility of 100 cm2 V−1 s−1 and a layer thickness of 200 nm for the MoSe2 layer. (iii) The use graphene transparent electrode can further enhance the carrier collection and thus reduce the carrier recombination. This is also evidenced by Figure 5e, in which the temporal responses of both the devices with and without graphene electrodes were measured. The former exhibits much faster light response than the latter. Furthermore, we studied the relative balance (Vmax–Vmin)/Vmax as a function of pulsed light frequency (Figure 5f). It is noteworthy that relative balance of the Gr/MoSe2/Si heterojunction photodetector decreases by less than 15% even at 10 kHz and remains more than 10% at 1 MHz. In contrast, relative balance of the device without graphene transparent electrode diminishes sharply and approximates to zero at 10 kHz.